Synthesis of ordered microporous activated carbons by chemical vapor deposition

ABSTRACT

Embodiments provide a methane microporous carbon adsorbent including a thermally-treated CVD carbon having a shape in the form of a negative replica of a crystalline zeolite has a BET specific surface area, a micropore volume, a micropore to mesopore volume ratio, a stored methane value and a methane delivered value and a sequential carbon synthesis method for forming the methane microporous carbon adsorbent. Introducing an organic precursor gas for a chemical vapor deposition (CVD) period to a crystalline zeolite that is maintained at a CVD temperature forms the carbon-zeolite composite. Introducing a non-reactive gas for a thermal treatment period to the carbon-zeolite composite maintained at a thermal treatment temperature forms the thermally-treated carbon-zeolite composite. Introducing an aqueous strong mineral acid mixture to the thermally-treated carbon-zeolite composite forms the methane microporous carbon adsorbent. The crystalline zeolite includes tri-ethanolamine (TEA) and has a shape that is orthogonal with a mid-edge length in a range of 8 μm to 20 μm.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. patentapplication Ser. No. 14/513,707, filed on Oct. 14, 2014, entitled“Synthesis of Ordered Microporous Activated Carbons by Chemical VaporDeposition,” which is hereby incorporated by reference in its entiretyinto this application.

BACKGROUND

Field

Embodiments relate to microporous carbons. More specifically,embodiments relate to the formation of microporous carbons and use innatural gas storage and transportation systems.

Description of the Related Art

Natural gas is the portable and preferred fuel of choice around theworld. Natural gas burns more completely than other traditional fuels,including petroleum and coal; therefore, the combustion of natural gasis comparatively less harmful to the environment. Natural gas andsimilar products, including LNG, propane and other compressed-gas fuels,are much more efficient in engine and turbine combustion systems.Pipelines are the traditional and most cost-effective means oftransporting natural gas from the producer to the consumer.

When producing electricity or natural gas for non-commercial users, asignificant problem arises for natural gas transportation networks:diurnal demand. People, unlike manufacturing plants or facilities, tendnot to be steady energy users throughout the day. People consume greateramounts of electricity during the day and into the early evening andmuch less at night and into the early morning. The higher rates ofconsumption form a “peak period of demand” and the lower rate ofconsumption creates a “non-peak period of demand”. This daily trendoccurs throughout the year. During different seasons, however, thelength of each period (longer or shorter periods of natural lightrequiring reduced or greater amounts of artificial light, respectively)and the amplitude of the period (for example, greater amounts demandedat higher and lower temperatures versus more moderate temperatures) canchange the amount of demand during the diurnal period. The location ofthe demand also has an impact upon the diurnal demand. In coolerenvironments, overall daily electrical and natural gas demand is lowerin the summer months and higher in winter months as consumers useheating equipment. In warmer environments, the daily demand trends areopposite as consumer use air conditioning units when it is hot.

Swinging electrical and natural gas consumption—not only in daily usebut also in seasonal differences—results in variability across thenatural gas transportation and production system. Natural gasproduction, however, is nearly constant. The supply-demand gap betweennatural gas production and total consumption results in a “gas demandlag”. The lag, without intervention, manifests itself as system pressureincreases and decreases (“swings”) across the natural gas transportationgrid.

Electrical generation facilities prefer constant, high-pressure naturalgas as a feedstock. Pressure swings in natural gas feed can damage theelectrical generation equipment, especially rotational equipment,including gas turbines, due to sudden inappropriate feed-to-fuel ratiosthat cause equipment slowdowns while under load.

A solution to mitigating the pressure swings in gas transportationnetworks is provided for in U.S. Pat. App. Pub. No. 2013/0283854(published Oct. 31, 2013) (Wang, et al.), titled “Adsorbed Natural GasStorage Facility”, which uses a microporous adsorbent to adsorb anddesorb natural gas.

Microporous adsorbents for adsorbed natural gas (ANG) storage includeactivated carbons, metal-organic frameworks (MOFs), zeolites and otherorganic or inorganic porous solids. MOFs have been reported to havesurface areas up to 4000 meters squared per gram (m²/g) and absolutemethane adsorption capacities as high as 230 volume to volume (v/v)absolute methane adsorption at 290 K and 35 bar (sometimes referred toas the “storage amount” ratio or the “amount stored” ratio). There issome question, however, as to whether this high number of absolutemethane adsorption is accurate. Several operational issues limit thepractical use of MOFs in natural gas adsorption-desorption systems.Methane, once adsorbed into the framework, is strongly bound, so fordesorption temperatures as high as 100° C. may be required to free theadsorbed methane. MOFs are known to have a reduced hydrothermalstability, so heating them to release methane repeatedly will eventuallydegrade the framework. MOFs also are intolerant to natural gasimpurities such as hydrogen sulfide, black/carbon-silicone powder andmercaptans, which are common in natural gas.

Metal oxide adsorbents such as zeolites tend to adsorb less methane thanactivated carbon materials at similar conditions. MOs possess a smallersurface area—reportedly less than about 800 m²/g. Zeolites also havehydrophilic surfaces relative to activate carbon material that makesthem adsorb water over other constituents in a natural gas stream.

Activated carbon materials have surface areas in a range up to about3000 m²/g and are relatively thermally and chemically stable materials.Activated carbons are known in the industry to have an absolute methaneadsorption capacity in a range of from about 130 to about 180 v/vmethane adsorption at 290 K and 35 bar.

There are several limitations to using activated carbon materials in anANG application. Activated carbon materials have generally a lowerpacking density than other materials due to the presence of meso-(2<d<50nanometers (nm)) and macro-sized pores (>50 nm). Larger micropores aregenerated upon formation of the activated carbon material due toexcessive carbon burn-off during the carbon activation process. Theirregular morphology of carbon particles with high surface areas tendsto cause the dense packing of particles, leaving voids betweenparticles. The optimum pore diameter for ANG is from about 1.1 to about1.2 nm. The meso- and macro-sized pores do not contribute to natural gasadsorption but do count as part of the material volume, resulting inlower packing density. Useful activated carbon materials have a bulkdensity is in a range of from about 0.20 to about 0.75 grams per cubiccentimeter (g/cm³).

Another issue is slow mass transport through microporous materials.Activated carbon materials having microporous can exhibit slow kineticadsorption-desorption behavior due to slow mass transport. Slow masstransport can be attributed to large micropore volumes withsmaller-than-useful pore diameters for adsorbing methane and a lack ofconnectivity between surface pore aperture openings (also known as “deadend pores”). Pressure and temperature changes can help accelerate themass transfer to and from the microporous material.

Another limitation is the number of potential materials useful to designthe activated carbon materials. Activated carbon materials are producedby chemical combustion of non-porous carbon precursors in a controlledmanner. Although this method provides an economic way of producingmaterial in the macro sense of a controlled reaction, rational andsystematic design of specific and regular carbon pore structures is notpossible due to the highly variable combustion process on the microlevel. Structure parameters including surface area, pore diameter andmicropore volume are strongly related to one another and are difficultto control separately. As an example, a high degree of burn-off achievesa large carbon surface area, which is positive for increasing gasstorage capacity. The high degree of burn-off, however, also results inthe unavoidable enlargement of pore diameters, which decreases theadsorption strength and packing density of the adsorbents per unitvolume.

It is desirable to develop a method for forming an activated carbonmaterial, the activated carbon material, and a method of its use thatmaintains or improves upon the packing density, the mass transport andthe adsorptive strength of activated carbon materials while maintainingor improving upon the surface area and absolute methane adsorptioncapacities of activated carbon materials. Ease of use and handling ofthe activated carbon material and simplicity of manufacturing are alsodesirable characteristics.

SUMMARY

A methane microporous carbon adsorbent including a thermally-treatedcarbon template of a crystalline zeolite having a shape in the form of anegative replica of the crystalline zeolite and has a BET specificsurface area, a micropore volume, a micropore to mesopore volume ratio,a stored methane value and a methane delivered value.

A sequential carbon synthesis method for forming a methane microporouscarbon adsorbent includes introducing an organic precursor gas made ofan organic precursor for a chemical vapor deposition (CVD) period to acrystalline zeolite that is maintained at a CVD temperature such thatthe carbon-zeolite composite forms. The introduced organic precursoradsorbs via CVD into the crystalline zeolite. The organic precursorconverts into carbon within the crystalline zeolite. The carbon withinthe crystalline zeolite forms a carbon template of the zeolite. Themethod includes introducing a non-reactive gas for a thermal treatmentperiod to the carbon-zeolite composite maintained at a thermal treatmenttemperature such that a thermally-treated carbon-zeolite compositeforms. The carbon template of the zeolite within the crystalline zeoliteconverts into a thermally-treated carbon template of the zeolite. Themethod includes introducing an aqueous strong mineral acid mixture tothe thermally-treated carbon-zeolite composite such that the methanemicroporous carbon adsorbent forms. The methane microporous carbonadsorbent is a negative replica of the crystalline zeolite, has a BETspecific surface area, a micropore volume, a micropore to mesoporevolume ratio, a stored methane value and a methane delivered value. Thecrystalline zeolite includes tri-ethanolamine (TEA) and has a shape thatis orthogonal with a mid-edge length in a range of 8 μm to 20 μm.

An embodiment of the method includes introducing the organic precursorgas for a second CVD period to the thermally-treated carbon-zeolitecomposite. The thermally-treated carbon-zeolite composite is maintainedat a second CVD temperature. A second carbon-zeolite composite forms.The organic precursor adsorbs via CVD into the thermally-treatedcarbon-zeolite composite. The organic precursor converts into carbonwithin the thermally-treated carbon-zeolite composite and forms with thethermally-treated carbon template of the zeolite a second carbontemplate of the zeolite. The embodiment of the method includesintroducing the non-reactive gas for a second thermal treatment periodto the second carbon-zeolite composite. The second carbon-zeolitecomposite is maintained at a second thermal treatment temperature. Thesecond thermally-treated carbon-zeolite composite forms. The secondcarbon template of the zeolite within the second carbon-zeolitecomposite converts into a second thermally-treated carbon template ofthe zeolite. In this embodiment of the method, the aqueous strongmineral acid mixture is introduced to the second thermally-treatedcarbon-zeolite composite instead of the thermally-treated carbon-zeolitecomposite.

The sequential carbon synthesis of the methane microporous carbonadsorbent uses both a chemical vapor deposition (CVD) and a post-thermaltreatment procedure for introducing, carbonizing and thermally treatingsmall organic compounds acting as carbon precursors in the pores of bothsmall and large zeolite crystals. The methane microporous carbonadsorbent has a microporous carbon structure that is the negativereplica of the zeolite structure in which it forms. The methanemicroporous carbon adsorbent has a well-defined micropore structure anda surface area similar to the sacrificial zeolite.

“Graphitizing” does not mean that the carbon-zeolite composite withinthe zeolite framework converts entirely into the graphite form ofcarbon. Complete dehydrogenation of the hydrocarbons and formation ofthe interlaced mono-carbon layer, carbon-carbon bonded 3-dimensionalstructures occurs at temperatures in excess of the temperatures used inthis process. Exposing the sacrificial zeolite framework tographitization temperatures would cause degradation of the zeolite.Temperatures greater than 1373 K are known to cause certain zeolitestructures to physically collapse in a short period of exposure. Rather,the deposited carbon forming the carbon template of the zeolite is morestrongly interconnected and rearranged into a matrix of stablecarbon-carbon bonds during thermal treatment as well as partiallydehydrogenated in the inert atmosphere. Therefore, if the term“graphitization” or its related conjugates are used, it is in the sensethat the deposition of carbon and the thermal treatment of the depositedcarbon induce an elevated level of dehydrogenation and the formation ofan interlacing carbon-carbon bonding network that is 3-dimensional butnot to the extent that a pure graphene network forms. This process ofdehydrogenation and interlacing occurs during both the deposition andthe thermal treatment periods.

An embodiment of the sequential carbon synthesis method includes CVD ofa zeolite while introducing an organic precursor at a CVD temperature ina range of from about 800 K to about 900 K for a CVD period in a rangeof from about 2 hours to about 9 hours. An embodiment of the methodincludes where the CVD is followed by a post-CVD thermal treatment at athermal treatment temperature in a range of from about 1100 K to about1200 K for a thermal treatment period of about 2 to 4 hours. In someembodiments of the method, the CVD/post-CVD thermal treatment cycles arerepeated. In some embodiments of such methods, the CVD and periodsbetween the first and the later cycles are different. The resultantthermally-treated carbon-zeolite composite is etched with an aqueousstrong mineral acid mixture to remove the sacrificial zeolite template.In an embodiment of the method, the strong mineral acid is selected fromthe group consisting of hydrochloric acid (HCl), hydrofluoric acid (HF),nitric acid (HNO₃), sulfuric acid (H₂SO₄) and combinations thereof. Inan embodiment of the method, the strong mineral acid is a combination ofHF and an acid selected from the group consisting of HCl, HNO₃, andH₂SO₄. In an embodiment of the method, the resultant thermally-treatedcarbon-zeolite composite is etched with a strong caustic tosacrificially remove the zeolite template. In an embodiment of such amethod, the strong caustic is aqueous sodium hydroxide (NaOH)_(aq). Theproduct methane microporous carbon adsorbent is the negative carbonreplica of the crystalline zeolite, which is an inverse carbon matrix ofthe zeolite network. In embodiments that perform at an additionalCVD/post-CVD thermal treatment cycle, the methane microporous carbonadsorbent has a more greatly ordered carbon structure that has anincreased BET specific surface area, greater micropore volume and areduced volume of mesopores than methane microporous carbon adsorbentsthat only go through one CVD/post-CVD thermal treatment cycle. Thisrepresents a greater amount of order in the thermally-treated carbontemplate of the zeolite. The densification of the deposited carbonduring the post-CVD thermal treatment forms strong carbon-carbonmolecularly bonded structures. The methane microporous carbon adsorbentthat is the negative carbon replica of the crystalline zeolite has amethane adsorption and a delivery capacity, and is suitable for use inANG storage operations.

In principle, the larger the BET specific surface area or the greaterthe micropore volume, the greater the methane adsorption capacity is fora given adsorbent. However, the methane adsorption capacity is alsoaffected by pore size distribution, micro-pore volume and packingdensity of the materials. As long as there is no diffusion limitation, ahigh fraction of micropore volume is better for methane storage.

The methane microporous carbon adsorbent is a negative carbon replica.The synthesis platform is a crystalline zeolite with microporestructures. The zeolite acts as a sacrificial template for forming themethane microporous carbon adsorbent. FIG. 1 shows a simplified schemeshowing the relationship between the crystalline zeolite and thenegative carbon replica that becomes the methane microporous carbonadsorbent. A microporous crystalline zeolite (a) is introduced for useas a sacrificial template. Small, high carbon:hydrogen ratio organicmolecules, including acetylene (1:1), propylene and ethylene (1:2), andethanol (1:3) are introduced into the crystalline zeolite. The organicmolecules are carbonized while inside the zeolite micropores, forming acarbon-zeolite composite (b). After carbon deposition, the zeoliteframework is removed by acid dissolution. The acid dissolution does notaffect the carbon template of the zeolite. The resultant (c) can be alarge, ordered methane microporous carbon adsorbent that is a negativereplica of the microporous zeolite.

An ANG storage facility for reducing the effect of diurnal demand on anatural gas source includes an adsorption bed system. The adsorption bedsystem has a methane storage capacity, contains a methane microporouscarbon adsorbent and is operable to both adsorb onto and desorb methanefrom the methane microporous carbon adsorbent. The ANG storage facilitycouples to a natural gas source such that natural gas is introduced intothe ANG storage facility and desorbed methane is introduced into thenatural gas source. Optionally, the natural gas storage facilityincludes a temperature control system and a compressor system.

A method of using the ANG storage facility includes introducing naturalgas into the ANG storage facility from a natural gas source during anon-peak period of demand such that the pressure within the natural gassource declines. The method includes the step of operating the ANGstorage facility during the non-peak period of demand such that methanemicroporous carbon adsorbent selectively separates methane from theintroduced natural gas and adsorbs the methane. The method includesmaintaining the ANG storage facility such that the adsorbed methaneremains adsorbed on the methane microporous carbon adsorbent until apeak period of demand. The method also includes the steps of operatingthe ANG storage facility during the period of peak demand such that themethane microporous carbon adsorbent desorbs the adsorbed methane. Themethod includes the step of introducing the desorbed methane into thenatural gas source during the period of peak demand such that thepressure within the natural gas source increases.

The adsorption natural gas storage facility is operable to receivenatural gas, to selectively separate methane from the introduced naturalgas and to store the methane via adsorption on the methane microporouscarbon adsorbent for a period. The ANG storage facility is also operableto desorb and release the adsorbed methane.

Introducing the methane to the natural gas source when natural gas is ingreater demand and removing methane from the natural gas source whennatural gas is not in demand reduces the amplitude of the pressureswings in the natural gas source, including a natural gas transportationsystem, caused by the difference between diurnal demand and steadynatural gas production.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the embodiments arebetter understood with regard to the following Detailed Description ofthe Preferred Embodiments, appended Claims, and accompanying Figures,where:

FIG. 1 shows a simplified scheme showing the relationship between (a)the crystalline zeolite and (b) the negative carbon replica that becomes(c) the methane microporous carbon adsorbent.

FIG. 2 is a cross-sectional diagram of a rotary tubular furnace in arotary tubular furnace system that is useful for batch-performingseveral steps of the sequential carbon synthesis method.

FIG. 3 is a process flow diagram of an embodiment of a system forcontinuously performing an embodiment of the sequential carbon synthesismethod.

FIG. 4 is a process flow diagram of an embodiment of the adsorbednatural gas (ANG) storage facility using the methane microporous carbonadsorbent.

FIG. 5 is a graph showing traces of the X-ray Diffraction (XRD) analysisfor each synthesized large crystal NaX zeolite.

FIG. 6 shows scanning electron micrographs (SEMs) of each synthesizedlarge NaX zeolite using TEA.

FIG. 7 is a graph showing traces of NH₃ temperature programmeddesorption (TPD) profiles of the ion-exchanged CaX zeolite and thecommercial-grade NaX zeolite.

FIG. 8 is a graph showing traces of the XRD analysis for several carbontemplates of the zeolite made using CaX zeolites.

FIG. 9 is a graph showing traces of nitrogen adsorption-desorptionisotherms of the carbon templates of the zeolite formed.

FIG. 10 is a graph showing traces of the determined pore sizedistribution using the non-local density function theory (NLDFT)algorithm on the nitrogen adsorption-desorption isotherm data shown inFIG. 9.

FIG. 11 is a graph showing traces of nitrogen adsorption-desorptionisotherms for the LCaX zeolite, a carbon-zeolite composite,thermally-treated carbon-zeolite composites, and the resultant methanemicroporous carbon adsorbent.

FIG. 12 is a graph showing traces of nitrogen adsorption-desorptionisotherms for carbon templates of the zeolite made from LCaX-1023-2 andLCaX-873-4 and two methane microporous carbon adsorbents made fromLCaX-873-4H and LCaX-873-4H4H.

FIG. 13 is a graph showing traces of the determined pore sizedistribution using the non-local density function theory (NLDFT)algorithm on the nitrogen adsorption-desorption isotherm data shown inFIG. 12.

FIG. 14 is a graph showing traces of the XRD analysis for carbontemplates of the zeolite made from LCaX-1023-2 and LCaX-873-4 andmethane microporous carbon adsorbents made from LCaX-873-4H andLCaX-873-4H4H.

FIG. 15 shows SEMs of (a) BEA crystalline zeolite and (b) methanemicroporous carbon adsorbents made using the BEA zeolite.

FIG. 16 is a graph showing traces of nitrogen adsorption-desorptionisotherms for the methane microporous carbon adsorbent made from CaX andBEA zeolites.

FIG. 17 is a graph showing traces of the XRD analysis for the methanemicroporous carbon adsorbent made from the CaX and BEA zeolites.

FIG. 18 is a graph showing traces of the determined pore sizedistribution using the NLDFT algorithm for four methane microporouscarbon adsorbents formed using two acetylene CVD/post-CVD thermaltreatment cycles.

FIG. 19 is a graph showing several traces of the methaneadsorption-desorption isotherms on a gravimetric basis for a carbontemplates of the zeolite and several methane microporous carbonadsorbents at 298 K.

FIG. 20 shows SEMs (a)-(b) of methane microporous carbon adsorbents madeusing calcium-ion substituted X zeolites.

FIG. 21 is a graph showing traces of nitrogen adsorption-desorptionisotherms for the methane microporous carbon adsorbents formed using CaXand NaX zeolites.

FIG. 22 is a graph showing traces of the determined pore sizedistribution using the non-local density function theory (NLDFT)algorithm on the nitrogen adsorption-desorption isotherm data shown inFIG. 21.

FIG. 23 shows SEMs (a)-(b) of methane microporous carbon adsorbents madeusing sodium X zeolites.

FIG. 24 is a graph showing traces of nitrogen adsorption-desorptionisotherms for the NaX and the mass produced NaX methane microporouscarbon adsorbents.

FIGS. 1-24 and their description facilitate a better understanding ofthe system and method of sequential carbon synthesis as well as thesystem and method for use of the adsorbed natural gas (ANG) storagefacility. In no way should FIGS. 1-24 limit or define the scope of theembodiments.

FIGS. 1-4 are simple diagrams for ease of description. Several of thegraphs show traces and curves that are off-set from the true y-axisvalue at y=0. This is done so for the sake of clarity and is indicatedin the Detailed Description and on each Figure.

DETAILED DESCRIPTION

The Specification, which includes the Summary, Brief Description of theDrawings and the Detailed Description, and the appended Claims refer toparticular features (including process or method steps). Those of skillin the art understand that the embodiments include all possiblecombinations and uses of particular features described in theSpecification. Those of skill in the art understand that the embodimentsare not limited to or by the description of embodiments given in theSpecification. The inventive subject matter is not restricted exceptonly in the spirit of the Specification and appended Claims.

Those of skill in the art also understand that the terminology used fordescribing particular embodiments does not limit the scope or breadth ofthe embodiments. In interpreting the Specification and appended Claims,all terms should be interpreted in the broadest possible mannerconsistent with the context of each term. All technical and scientificterms used in the Specification and appended Claims have the samemeaning as commonly understood by one of ordinary skill in the art towhich the embodiments belong unless defined otherwise.

As used in the Specification and appended Claims, the singular forms“a”, “an” and “the” include plural references unless the context clearlyindicates otherwise. The verb “comprises” and its conjugated formsshould be interpreted as referring to elements, components or steps in anon-exclusive manner, and the embodiments illustrative disclosedsuitably may be practiced in the absence of any element which is notspecifically disclosed, including as “consisting essentially of” and“consisting of”. The referenced elements, components or steps may bepresent, utilized or combined with other elements, components or stepsnot expressly referenced. “Operable” and its various forms means fit forits proper functioning and able to be used for its intended use.“Detect” and its conjugated forms should be interpreted to mean theidentification of the presence or existence of a characteristic orproperty. “Determine” and its conjugated forms should be interpreted tomean the ascertainment or establishment through analysis or calculationof a characteristic or property.

Spatial terms describe the relative position of an object or a group ofobjects relative to another object or group of objects. The spatialrelationships apply along vertical and horizontal axes. Orientation andrelational words, including “upstream” and “downstream” and other liketerms are for descriptive convenience and are not limiting unlessotherwise indicated.

Where the Specification or the appended Claims provide a range ofvalues, it is understood that the interval encompasses each interveningvalue between the upper limit and the lower limit as well as the upperlimit and the lower limit. Embodiments encompass and bounds smallerranges of the interval subject to any specific exclusion provided.

Where the Specification and appended Claims reference a methodcomprising two or more defined steps, the defined steps can be carriedout in any order or simultaneously except where the context excludesthat possibility.

When a patent or a publication is referenced in this disclosure, thereference is incorporated by reference and in its entirety to the extentthat it does not contradict statements made in this disclosure.

FIG. 2 is a cross-sectional diagram of a rotary tubular furnace in arotary tubular furnace system. The rotary tubular furnace system isuseful for batch-performing several steps of the sequential carbonsynthesis method. Rotary tubular furnace system 100 includes rotarytubular furnace 110. Rotary tubular furnace 110 contains cylindricalstainless-steel container 112. Cylindrical stainless-steel container 112has several interior baffles 114 mounted along the interior surface forthe length of cylindrical stainless-steel container 112. The rotation ofcylindrical stainless-steel container 112 causes interior baffles 114 tocontact and collide with solid material 116 previously introduced intocylindrical stainless-steel container 112. The repeated contacting andcollision with interior baffles 114 distributes solid material 116 in arandom manner within the interior of cylindrical stainless-steelcontainer 112 that facilitates solid-gas mixing, distributes heat andprevents solids from adhering to one another. Cylindricalstainless-steel container 112 couples to inlet gas tubular 118 at firstcircular end 120 and outlet gas tubular 122 at second circular end 124.During operation, cylindrical stainless-steel container 112 rotatesaround a lengthwise axis (arrows 126) formed by the coupling of inletgas tubular 118, cylindrical stainless-steel container 112 and outletgas tubular 122.

Rotary tubular furnace 110 contains cylindrical stainless-steelcontainer 112 within cylindrical shell 128. Heating units 130 are fixedalong the exterior of the lengthwise portion of cylindrical shell 128such that they are operable to transmit thermal energy into the interiorof cylindrical shell 128. Thermocouples 132 are located in heating units130, on cylindrical shell 128, and inside cylindrical shell 128.Temperature controller 134, which electrically couples to heating units130 and signally to thermocouples 132, is operable to monitor thetemperature values provided by thermocouples 132 and adjust thetransmission of thermal energy into cylindrical shell 128 such thatrotary tubular furnace system 100 is operable to maintain a settemperature for cylindrical stainless-steel container 112 duringoperation.

Rotary tubular furnace system 100 includes organic precursor source 136and non-reactive gas source 138. Rotary tubular furnace system 100 isoperable to selectively feed organic precursor source 136, non-reactivegas source 138 or both simultaneously to rotary tubular furnace 110.Mixer 140 is downstream of both organic precursor source 136 andnon-reactive gas source 138 and is operable to blend the two sourcegases together into a homogeneous mixture when both are introducedsimultaneously.

Using rotary tubular furnace system 100 to perform the sequential carbonsynthesis includes introducing a crystalline zeolite into cylindricalstainless-steel container 112. The crystalline zeolite acts as asacrificial template for forming the methane microporous carbonadsorbent. Cylindrical shell 128 is closed such that it is air tight.Cylindrical stainless-steel container 112 is set in rotation (arrows126) such that the crystalline zeolite is mixed using the interiorbaffles 114. Non-reactive gas is introduced into cylindricalstainless-steel container 112 from non-reactive gas source 138 throughinlet gas tubular 118 to purge the atmosphere within cylindricalstainless-steel container 112 through outlet gas tubular 122 and fillcylindrical stainless-steel container 112 with non-reactive gas. A firstchemical vapor deposition (CVD) temperature is set using temperaturecontroller 134, which raises in a steady and controlled manner thetemperature of cylindrical stainless-steel container 112 until the firstCVD temperature is detected through thermocouples 132.

Upon achieving the first CVD temperature, a mixture of the organicprecursor and the non-reactive gas is introduced into cylindricalstainless-steel container 112 to fill and maintain cylindricalstainless-steel container 112 with the first CVD gas mixture. Theorganic precursor gas is introduced into cylindrical stainless-steelcontainer 112 through inlet gas tubular 118 from organic precursorsource 136 after mixing with non-reactive gas in mixer 140. Thecrystalline zeolite is exposed to the first CVD gas mixture for a firstCVD period at the first CVD temperature such that the introduced organicprecursor is adsorbed via CVD into the crystalline zeolite, the organicprecursor converts into a deposited carbon that negatively replicatesthe crystalline zeolite, and a first carbon-zeolite composite forms.

After the elapse of the first CVD period, a first thermal treatmenttemperature is set using temperature controller 134 to raise theoperating temperature of the cylindrical stainless-steel container 112to the first thermal treatment temperature. In addition, the organicprecursor source 136 is isolated such that only the non-reactive gas isintroduced into cylindrical stainless-steel container 112. Uponachieving the first thermal treatment temperature, the introduction ofnon-reactive gas and the first thermal treatment temperature ismaintained for a first thermal treatment period. During the firstthermal treatment period, the deposited carbon within the carbon-zeolitecomposite converts into a thermally-treated carbon that negativelyreplicates the crystalline zeolite, and the first thermally-treatedcarbon template of the zeolite forms.

The processes of CVD and post-CVD thermal treatment are repeated suchthat a second thermally-treated carbon template of the zeolite forms.The CVD occurs for a second CVD period at the second CVD temperature.The post-CVD thermal treatment occurs at a second thermal treatmenttemperature for a second thermal treatment period. After expiration ofthe second thermal treatment period, the introduction of non-reactivegas continues and cylindrical stainless-steel container 112 is permittedto cool to room temperature. Upon reaching room temperature, therotation of cylindrical stainless-steel container 112 is halted andsolid material 116, which includes second thermally-treatedcarbon-zeolite composite, is recovered from rotary tubular furnacesystem 100.

After recovery of the second thermally-treated carbon-zeolite composite,an aqueous strong mineral acid mixture is introduced to the secondthermally-treated carbon-zeolite composite. The aqueous strong mineralacid mixture etches the crystalline zeolite away from the secondthermally-treated carbon template of the zeolite, forming the methanemicroporous carbon adsorbent.

FIG. 3 is a process flow diagram of an embodiment of a system forcontinuously performing an embodiment of the sequential carbon synthesismethod. Sequential carbon synthesis system 200 includes firstCVD/thermal treatment system 202 (dotted box), second CVD/thermaltreatment system 204 (dotted box) and recovery system 206 (dotted box).First CVD/thermal treatment system 202 and second CVD/thermal treatmentsystem 204 are coupled in series. First CVD/thermal treatment system 202is operable to form the first thermally-treated carbon-zeolite compositefrom the introduced crystalline zeolite, and second CVD/thermaltreatment system 204 is operable to form the second thermally-treatedcarbon-zeolite composite using the first thermally-treatedcarbon-zeolite composite. Recovery system 206 is operable to form themethane microporous carbon adsorbent from the first thermally-treatedcarbon-zeolite composite, the second thermally-treated carbon-zeolitecomposite, and combinations thereof.

Several streams are introduced into sequential carbon synthesis system200 to support the formation of the negative replica that is the methanemicroporous carbon adsorbent. Sequential carbon synthesis system 200processes crystalline zeolite introduced through first feed line 210from a source outside of the process into a thermally-treatedcarbon-zeolite composite. CVD gas supply lines 212 and 214 introducefrom a source outside of the process a gas that includes the organicprecursor. Acetylene, ethylene, propylene and ethanol are useful organicprecursors. In an embodiment of the method, the organic precursor isselected from the group consisting of acetylene, ethylene, propylene,ethanol and combinations thereof. In an embodiment of the method, theorganic precursor gas further comprises a non-reactive gas. Neutral gassupply lines 216 and 218 introduce from a source outside of the processa gas that has no reactivity with the carbon-zeolite composite duringpost-CVD thermal treatment at the thermal treatment temperature.Non-reactive gases include noble gases such as helium and argon. In anembodiment of the method, the non-reactive gas is selected from thegroup consisting of helium, argon and combinations thereof. Acid supplyline 220 introduces from a source outside of the process an aqueousstrong mineral acid mixture for removing the sacrificial crystallinezeolite from a thermally-treated carbon-zeolite composite. Aqueousstrong mineral acid mixture includes aqueous mixtures of HCl and HF.

The produced methane microporous carbon adsorbent—a negative carbonreplica of the introduced crystalline zeolite—passes from sequentialcarbon synthesis system 200 through adsorbent product line 222. Severalstreams also pass from sequential carbon synthesis system 200 asbyproducts of the process. Spent CVD gas recovery lines 224 and 226direct recovered CVD gas, which contains unused organic precursor andhydrogen from the carbonization of the organic precursor, to systemsoutside of the process for separation and recovery. The recovered CVDgas may also contain a non-reactive gas worth recovering in processesoutside of sequential carbon synthesis system 200. Helium is ahighly-limited natural resource that many countries consider a strategicmaterial. Argon is a useful non-reactive gas and is easy to separatefrom the organic species. Neutral gas recovery lines 228 and 230 directa mixture of non-reactive gas introduced through neutral gas supplylines 216 and 218 and hydrogen from post-CVD thermal treatment tosystems outside of the process for recovery and purification. Spent acidrecovery line 232 passes a spent aqueous strong mineral acid mixture tosystems outside of the process for regeneration or neutralization. Thespent aqueous strong mineral acid mixture contains dissolved aluminumand silicon from etching the sacrificial zeolite to form the methanemicroporous carbon adsorbent.

Sequential carbon synthesis system 200 introduces crystalline zeoliteinto first zeolite hopper 240 of first CVD/thermal treatment system 202through first feed line 210. First zeolite hopper 240 couples to andmeters the crystalline zeolite into first CVD reactor 242 using solidsfeed line 244. Sequential carbon synthesis system 200 introduces theorganic precursor into the first CVD reactor 242 as part of an organicprecursor gas using CVD gas supply line 212. In an embodiment of themethod, the organic precursor gas includes a non-reactive gas. First CVDreactor 242 can be a number of known reactor types for mixing solids andgases together where the solids require a certain residence time withinthe reactor, including a moving bed type reactor or a fluidized bedreactor. First CVD reactor 242 is shown with perforated plate 246(dashed line) such that the organic precursor gas is introduced throughCVD gas supply line 212 below the stack of crystalline zeolites (notshown), which are in various stages of adsorption and carbonization. Theorganic precursor gas moves upward from the bottom of first CVD reactor242 to the top, interacting with the introduced zeolite. The formedfirst carbon-zeolite composite passes from first CVD reactor 242 throughcarbonized composite line 248. The spent CVD gas passes from the top offirst CVD reactor 242 through spent CVD gas line 250. Spent CVD gas line250 couples to and feeds into spent CVD gas recovery line 224.

Sequential carbon synthesis system 200 operates first CVD reactor 242such that the introduced organic precursor is adsorbed via chemicalvapor deposition (CVD) into the crystalline zeolite, the organicprecursor converts into a deposited carbon that negatively replicatesthe crystalline zeolite, and the first carbon-zeolite composite forms.Sequential carbon synthesis system 200 maintains first CVD reactor 242at a first CVD temperature. In an embodiment of the method, the firstCVD temperature is in a range of from about 800 K to about 900 K.Sequential carbon synthesis system 200 maintains the crystalline zeolitewithin first CVD reactor 242 for a first CVD period. In an embodiment ofthe method, the first CVD period is in range of from about 2 hours toabout 9 hours.

Carbonized composite line 248 couples first CVD reactor 242 to purgevessel 252 and conveys the first carbon-zeolite composite into purgevessel 252. Sequential carbon synthesis system 200 operates purge vessel252 to remove any remaining organic precursor from the firstcarbon-zeolite composite for recovery and reuse. Any recovered organicprecursor is conveyed to spent CVD gas recovery line 224 throughrecovered gas line 254. In an embodiment of the method, the purge vesselis maintained at a sub-atmospheric pressure. A gas that is non-reactivewith the first carbon-zeolite composite can be introduced to purge thefirst carbon-zeolite composite, including helium and argon. In anembodiment of the method, a purge gas that is non-reactive with thefirst carbon-zeolite composite is introduced into the purge vessel.Sequential carbon synthesis system 200 passes the purged firstcarbon-zeolite composite from purge vessel 252 using first thermaltreatment feed line 256.

First thermal treatment feed line 256 couples purge vessel 252 to firstpost-CVD thermal treatment unit 258. Sequential carbon synthesis system200 introduces the purged first carbon-zeolite composite into firstpost-CVD thermal treatment unit 258. Neutral gas supply line 216introduces the non-reactive gas into first post-CVD thermal treatmentunit 258. First post-CVD thermal treatment unit 258 can be a number ofknown reactor types for mixing solids and gases together where thesolids require a certain residence time within the reactor as previouslydescribed. The formed first thermally-treated carbon-zeolite compositepasses from first post-CVD thermal treatment unit 258 through firsttreatment product line 268. In doing so, the first thermally-treatedcarbon-zeolite composite passes from first CVD/thermal treatment system202. The spent thermal treatment gas, which comprises non-reactive gasas well as evolved hydrogen from the thermal treatment of thecarbon-zeolite composite, passes from the top of first post-CVD thermaltreatment unit 258 through neutral gas recovery line 228.

Sequential carbon synthesis system 200 operates first post-CVD thermaltreatment unit 258 such that the carbon template of the zeolite withinthe first carbon-zeolite composite converts into a thermally-treatedcarbon template of the zeolite that negatively replicates the zeolite.Sequential carbon synthesis system 200 maintains first post-CVD thermaltreatment unit 258 at a first thermal treatment temperature. In anembodiment of the method, the first thermal treatment temperature is ina range of from about 1100 K to about 1200 K. Sequential carbonsynthesis system 200 maintains the purged first carbon-zeolite compositewithin first post-CVD thermal treatment unit 258 for a first thermaltreatment period. In an embodiment of the method, the first thermaltreatment period is in range of from about 2 hours to about 4 hours.

Sequential carbon synthesis system 200 includes solids feeds splitter270, which is operable to selectively direct the first thermally-treatedcarbon-zeolite composite received from coupled first treatment productline 268 towards zeolite reactor 272 via reactor feed line 274 ortowards second zeolite hopper 340 via second feed line 310, or both inproportion at the same time. Solids feeds splitter 270 is operable todirect none, at least a portion of the first thermally-treatedcarbon-zeolite composite towards zeolite reactor 272 and the remainder,if any, towards second zeolite hopper 340.

The inclusion of a splitter allows the flexibility to operate thesequential carbon synthesis system to form methane microporous carbonadsorbents using only the first CVD/thermal treatment system, the secondCVD/thermal treatment systems, or both simultaneously.

Recovery system 206 of sequential carbon synthesis system 200 includeszeolite reactor 272. Reactor feed line 274 couples purge vessel 264 tozeolite reactor 272 and conveys the first thermally-treatedcarbon-zeolite composite into zeolite reactor 272. Sequential carbonsynthesis system 200 introduces the aqueous strong mineral acid mixtureinto zeolite reactor 272 through acid supply line 220. Zeolite reactor272 is operable to form the methane microporous carbon adsorbent.Zeolite reactor 272 etches the crystalline zeolite of the firstthermally-treated carbon-zeolite composite from the thermally-treatedcarbon template of the zeolite using the aqueous strong mineral acidmixture. In an embodiment of the method, the first thermally-treatedcarbon-zeolite composite is maintained within the zeolite rector for aresidence time in a range of from about an hour to about two hours. Inan embodiment of the method, the aqueous strong mineral acid mixturecomprises HCl and HF. The aqueous strong mineral acid mixture uponreaction with the crystalline zeolite converts into the spent aqueousstrong mineral acid mixture. Sequential carbon synthesis system 200passes the suspension of methane microporous carbon adsorbent and spentaqueous strong mineral acid mixture from zeolite reactor 272 usingzeolite reactor product line 276.

Recovery system 206 also includes adsorbent recovery unit 278. Zeolitereactor product line 276 couples zeolite reactor 272 to adsorbentrecovery unit 278 and conveys the suspension of methane microporouscarbon adsorbent and spent aqueous strong mineral acid mixture intoadsorbent recovery unit 278. Adsorbent recovery unit 278 is operable toseparate the suspension into the methane microporous carbon adsorbentand the spent aqueous strong mineral acid mixture. The produced methanemicroporous carbon adsorbent passes through adsorbent product line 222,and spent acid recovery line 232 passes the spent aqueous strong mineralacid mixture.

Sequential carbon synthesis system 200 introduces the firstthermally-treated carbon-zeolite composite into second CVD/thermaltreatment system 204 using second zeolite hopper 340 through second feedline 310. Second zeolite hopper 340 couples to and meters the firstthermally-treated carbon-zeolite composite into second CVD reactor 342using solids feed line 344. Sequential carbon synthesis system 200introduces the organic precursor into the second CVD reactor 342 as partof an organic precursor gas using CVD gas supply line 214. In anembodiment of the method, the organic precursor gas includes anon-reactive gas. Second CVD reactor 342 can be a number of knownreactor types for mixing solids and gases together where the solidsrequire a certain residence time within the reactor. Second CVD reactor342 has perforated plate 246 (dashed line) such that the organicprecursor gas is introduced through CVD gas supply line 214 below thestack of thermally-treated carbon-zeolite composite (not shown), whichare in various stages of adsorption and carbonization. The organicprecursor gas moves upward from the bottom of second CVD reactor 342 tothe top, interacting with the introduced first thermally-treatedcarbon-zeolite composite. The formed second carbon-zeolite compositepasses from second CVD reactor 342 through carbonized composite line348. The spent CVD gas passes from the top of second CVD reactor 342through spent CVD gas line 350. Spent CVD gas line 350 couples to andfeeds into spent CVD gas recovery line 226.

Sequential carbon synthesis system 200 operates second CVD reactor 342such that the introduced organic precursor is adsorbed via CVD into thefirst thermally-treated carbon-zeolite composite, the organic precursorconverts into carbon and the second carbon-zeolite composite forms. Thefirst thermally-treated carbon-zeolite composite already contains thefirst thermally-treated carbon template of the zeolite from the firstCVD/post-CVD thermal treatment. The newly deposited carbon furtherenhances the accuracy of the negative carbon replica, although thedeposited carbon is not fully incorporated into the existing firstthermally-treated carbon template of the zeolite at the lower CVDtemperatures. Sequential carbon synthesis system 200 maintains secondCVD reactor 342 at a second CVD temperature. In an embodiment of themethod, the second CVD temperature is in a range of from about 800 K toabout 900 K. In an embodiment of the method, the first CVD temperatureand the second CVD temperature are the same. Sequential carbon synthesissystem 200 maintains the first thermally-treated carbon-zeolitecomposite within second CVD reactor 342 for a second CVD period. In anembodiment of the method, the second CVD period is in range of fromabout 2 hours to about 4 hours. In an embodiment of the method, thefirst CVD period and the second CVD period are the same.

Carbonized composite line 348 couples second CVD reactor 342 to purgevessel 352 and conveys the second carbon-zeolite composite into purgevessel 352. Sequential carbon synthesis system 200 operates purge vessel352 in a similar manner as purge vessel 252. Any recovered organicprecursor is conveyed to spent CVD gas recovery line 226 throughrecovered gas line 354. In an embodiment of the method, the purge vesselis maintained at a sub-atmospheric pressure. In an embodiment of themethod, a purge gas that is non-reactive with the second carbon-zeolitecomposite is introduced into the purge vessel. Sequential carbonsynthesis system 200 passes the purged second carbon-zeolite compositefrom purge vessel 352 using second thermal treatment feed line 356.

Second thermal treatment feed line 356 couples purge vessel 352 tosecond post-CVD thermal treatment unit 358 and introduces the purgedsecond carbon-zeolite composite into second post-CVD thermal treatmentunit 358. Sequential carbon synthesis system 200 introduces thenon-reactive gas into second post-CVD thermal treatment unit 358 usingneutral gas supply line 218. Second post-CVD thermal treatment unit 358can be a number of known reactor types for mixing solids and gasestogether where the solids require a certain residence time within thereactor as previously described. The formed second thermally-treatedcarbon-zeolite composite passes from second post-CVD thermal treatmentunit 358 through thermally-treated carbon-zeolite composite line 360.The spent thermal treatment gas, which comprises non-reactive gas aswell as evolved hydrogen, passes from the top of second post-CVD thermaltreatment unit 358 through spent thermal treatment gas line 362. Spentthermal treatment gas line 362 couples to and feeds into neutral gasrecovery line 230.

Sequential carbon synthesis system 200 operates second post-CVD thermaltreatment unit 358 such that the deposited carbon within the secondcarbon-zeolite composite converts into a thermally-treated carbontemplate of the zeolite that is the negative replica. The secondthermally-treated carbon-zeolite composite forms as a result. Thedeposited carbon from the second CVD period is fully incorporated duringthe second thermal treatment period into the first thermally-treatedtemplate of the zeolite, thereby forming the second thermally-treatedtemplate of the zeolite. In addition, additional calcination timeprovides energy to the existing negative carbon replica to improve itsconformance to the crystalline zeolite structure, further improving theaccuracy of the negative replication. Sequential carbon synthesis system200 maintains second post-CVD thermal treatment unit 358 at a secondthermal treatment temperature. In an embodiment of the method, thesecond thermal treatment temperature is in a range of from about 1100 Kto about 1200 K. In an embodiment of the method, the second thermaltreatment temperature is the same as the first thermal treatmenttemperature. Sequential carbon synthesis system 200 maintains the purgedsecond carbon-zeolite composite within second post-CVD thermal treatmentunit 358 for a second thermal treatment period. In an embodiment of themethod, the second thermal treatment period is in range of from about 2hours to about 4 hours. In an embodiment of the method, the secondthermal treatment period is the same as the first thermal treatmentperiod.

Thermally-treated carbon zeolite composite line 260 couples secondpost-CVD thermal treatment unit 358 to purge vessel 364 and conveys thesecond thermally-treated carbon-zeolite composite into purge vessel 364.Sequential carbon synthesis system 200 operates purge vessel 364 toremove any remaining non-reactive gas and evolved hydrogen from secondthermally-treated carbon-zeolite composite. The degassing mitigates theneed for gas recovery for zeolite reactor 272, which operates usingstrong acids that can partially volatilize at room conditions. Anyrecovered gas is conveyed to neutral gas recovery line 230 throughrecovered gas line 366. In an embodiment of the method, the purge vesselis maintained at a sub-atmospheric pressure. Sequential carbon synthesissystem 200 passes the purged second thermally-treated carbon-zeolitecomposite from purge vessel 364 using second treatment product line 368.In doing so, the second thermally-treated carbon-zeolite compositepasses from second CVD/thermal treatment system 204.

Second treatment product line 368 couples purge vessel 364 to zeolitereactor 272 and conveys the purged second thermally-treatedcarbon-zeolite composite into zeolite reactor 272. Zeolite reactor 272is operable to form the methane microporous carbon adsorbent from thesecond thermally-treated carbon-zeolite composite using the aqueousstrong mineral acid mixture. The methane microporous carbon adsorbentforms by etching the crystalline zeolite of the second thermally-treatedcarbon-zeolite composite from the thermally-treated carbon template ofthe zeolite. In an embodiment of the method, the secondthermally-treated carbon-zeolite composite is maintained within thezeolite rector for a residence time in a range of from about an hour toabout two hours. The aqueous strong mineral acid mixture converts intothe spent aqueous strong mineral acid mixture upon reacting with thezeolite. Sequential carbon synthesis system 200 passes the suspension ofmethane microporous carbon adsorbent and spent aqueous strong mineralacid mixture from zeolite reactor 272 using zeolite reactor product line276.

FIG. 4 is a process flow diagram of an embodiment of the adsorbednatural gas (ANG) storage facility using the methane microporous carbonadsorbent. Natural gas storage facility 400 couples to compressednatural gas (CNG) transportation pipeline 402, which is a natural gassource, at upstream connection 404 and downstream connection 406.Upstream isolation valve 408 and downstream isolation valve 410 areoperable to fluidly isolate natural gas storage facility 400 from CNGtransportation pipeline 402. Check valves 412 provide additionalassurance that any fluid flowing through natural gas storage facility400 is one-way from upstream connection 404 to downstream connection406.

Solar power array 424 electrically couples using electrical powerconduit 426 to temperature control system 428. Solar power array 424provides electrical power such that temperature control system 428satisfies the temperature regulation requirements of natural gas storagefacility 400 during both the peak period of demand and the non-peakperiod of demand.

Natural gas introduced into natural gas storage facility 400 passesthrough adsorption beds inlet isolation valve 440 into adsorption bedsystem 436. Adsorption bed system 436 has several separate adsorptionbed 438 in parallel. Each adsorption bed 438 contains the methanemicroporous carbon adsorbent (not shown) for retaining the light naturalgas component during the non-peak period of demand. Adsorption bedsthermal jacket 442 surrounds the exterior of and regulates the internaltemperature of adsorption bed 438. Desorbed methane passes fromadsorption bed 438 through adsorption beds outlet isolation valve 444.

Temperature control system 428 couples to adsorption beds thermal jacket442. Temperature control system 428 controls, maintains and modifies theinternal temperature of adsorption bed 438. Temperature control system428 introduces a temperature-modifying fluid into adsorption bedsthermal jacket 442 via adsorption bed supply conduit 446. Heat transfersto and from the temperature-modifying fluid in adsorption beds thermaljacket 442 to support the adsorption and desorption of methane from themethane microporous carbon adsorbent (not shown) contained in severaladsorption bed 438. Spent temperature modifying fluid returns fromadsorption beds thermal jacket 442 via adsorption bed return conduit448.

Adsorption bed system 436 couples to storage facility compressor 450 andcompressor bypass valve 452 via adsorption beds outlet isolation valve444. Both storage facility compressor 450 and compressor bypass valve452 provide access to CNG transportation pipeline 402 from adsorptionbed 438. Storage facility compressor 450 is operable to pressurize andintroduce the desorbed methane into CNG transportation pipeline 402through discharge conduit 456. Compressor bypass valve 452 permitsdirect connectivity between adsorption bed system 436 and CNGtransportation pipeline 402 through discharge conduit 456. Storagefacility compressor 450 is operable of reduce the pressure in adsorptionbed 438 to facilitate desorption and purging of adsorbed methane fromthe methane microporous adsorbent.

During the non-peak period of demand, a detectable condition triggersnatural gas storage facility 400 to operate the isolation valves,including upstream isolation valve 408, adsorption beds inlet isolationvalve 440 and downstream isolation valve 410, such that a fluid pathwayforms through natural gas storage facility 400. Pressure differencesbetween CNG transportation pipeline 402 and adsorption bed system 436motivates natural gas to flow from CNG transportation pipeline 402 intoseveral adsorption bed 438 of adsorption bed system 436. At reducedtemperatures and increasing pressure (as more natural gas flows intonatural gas storage facility 400), the methane from the introducednatural gas is selectively separated and adsorbed by the methanemicroporous carbon adsorbent in adsorption bed system 436. The remainderflows out of natural gas storage facility 400 back into CNGtransportation pipeline 402 via discharge conduit 456. Temperaturecontrol system 428 supplies temperature-modifying fluid to adsorptionbeds thermal jacket 442 to facilitate the selective separation andadsorption of methane by the methane microporous carbon adsorbent inseveral adsorption bed 438.

Either at the end of the non-peak period of demand or when some otherdetectable condition is detected, upstream isolation valve 408,adsorption beds inlet isolation valve 440, adsorption beds outletisolation valve 444 and downstream isolation valve 410 close to isolatenatural gas storage facility 400 from CNG transportation pipeline 402.Temperature control system 428 maintains a storage temperature inadsorption bed system 436 such that the adsorbed methane remainsadsorbed onto the methane microporous carbon adsorbent in adsorption bed438 until the peak period of demand.

During the peak period of demand, a condition detected by natural gasstorage facility 400 triggers it to operate adsorption beds outletisolation valve 444 and downstream isolation valve 410 such that a fluidpathway forms between CNG transportation pipeline 402 and adsorption bedsystem 436. Temperature control system 428 operates such that thetemperature-modifying fluid is provided to adsorption beds thermaljacket 442. The temperature-modifying fluid facilitates desorption ofadsorbed methane from the methane microporous carbon adsorbent in theseveral adsorption bed 438, forming desorbed methane.

Introduction of the desorbed methane into CNG transportation pipeline402 from adsorption bed 438 at times during the peak period of demandoptionally occurs without the need of compressive assistance by openingcompressor bypass valve 452. If a suitable differential pressure betweenadsorption bed 438 and CNG transportation pipeline 402 exists thedesorbed methane optionally flows from adsorption bed system 436 intoCNG transportation pipeline 402 without compression. When compression isused, closing compressor bypass valve 452 and operating storage facilitycompressor 450 provides motivation to the desorbed methane forintroduction into CNG transportation pipeline 402 during the peak periodof demand.

The operation of storage facility compressor 450 is operable to form asub-atmospheric pressure or “partial vacuum” in the several adsorptionbed 438 to facilitate desorption of methane in preparation for the nextadsorption cycle. Closing adsorption beds outlet isolation valve 444during the partial vacuum condition causes adsorption bed 438 to retainthe sub-atmospheric pressure condition, which facilitates additionaldesorption of methane.

Natural gas storage facility 400 operations also includes operatingtemperature control system 428 such that it provides heating or cooling,shutting down storage facility compressor 450, opening isolation valvesfor adsorption bed 438 to equalize pressure and closing all otherremaining isolation valves to natural gas storage facility 400.

Experiments

Several experiments show the formation of methane microporous carbonadsorbents from commercial-grade and large crystalline zeolites. Usefulmethane microporous carbon adsorbents are manufactured using a varietyof crystalline zeolites, organic precursors, CVD temperatures andperiods, and post-CVD treatment temperatures and periods. The variety ofuseful materials shows the versatility of the sequential carbonsynthesis method in forming a relatively high surface area and microporevolume methane microporous carbon adsorbents.

Synthesis of Large Crystal NaX Zeolites (Si:Al=1.35-1.45)

This experiment shows the formation of “large” (versus commercial-gradesizes of 1 to 2 μm) crystal NaX zeolites. The large NaX zeolites areuseful to act as the sacrificial framework for forming the methanemicroporous carbon adsorbent. In an embodiment of the method, the methodfurther comprises the step of forming the crystalline zeolite.

In an embodiment of the method, the crystalline zeolite comprisestri-ethanolamine (TEA). Large crystal NaX zeolites are synthesized byadding TEA into a zeolite synthesis gel. Na₂SiO₃.5H₂O and sodiumaluminate (55% Al₂O₃ and 45% Na₂O) are used as the silica and aluminaprecursors, respectively. TEA is a complexing agent for aluminumcations. The presence of TEA can retard the nucleation of zeolitecrystals compared with the growth process of the crystal, resulting inlarger crystals when included. The resultant gel composition is in amolar ratio of about 4.76 Na₂O:1.0 Al₂O₃:3.5 SiO₂:454 H₂O:n TEA, where nis varied at three values for forming three different test gels: About3, 5 and 7. Each resultant test gel is transferred to a polypropylenebottle and hydrothermally crystallized at 373 K (Kelvin) for 72 hours.Each large NaX zeolite product is collected by filtration and dried at373 K.

FIG. 5 is a graph showing traces of the X-ray Diffraction (XRD) analysisfor each of the synthesized large NaX zeolites. Note that for the sakeof clarity in FIG. 5 that the individual traces of NaX-7TEA and NaX-5TEAare offset by a fixed value of Intensity in counts per second (CPS). Inreality, all three traces have the same value at 2θ=0. The trace forNaX-5TEA is offset by 6700 CPS; the trace for NaX-7TEA is offset by12000 CPS. XRD analysis of each large NaX zeolite shows that the NaXzeolite made with TEA (n=3) is the only large NaX zeolite with XRD trace“peaks” corresponding to those of a traditional NaX zeolite. No otherimpurity phase peaks are observed with this zeolite. In contrast, bothlarge NaX zeolites made with TEA (n=5, 7) show XRD trace peakscorresponding to a “P” zeolite (GIS phase) (see arrows FIG. 5) thatsupplement the NaX zeolite phase peaks. The results indicate that usingless TEA as part of the gel composition is more useful for forming largecrystal NaX zeolites than greater amounts of the compound.

FIG. 6 shows SEMs of each synthesized large NaX zeolite using TEA. TheSEMs reveal the size of the zeolite framework. Each of the SEMs (a)-(c)of FIG. 6 shows the large NaX zeolites made with TEA having the typicalorthogonal crystal morphology of a NaX zeolite (approximately octahedralin geometry—FAU) but with a crystal size distribution (left portion ofSEMs (a)-(c) of FIG. 6) such that the mid-edge length of the octahedronis in the range of about 8 μm (micrometers or microns) to about 20 μm.In an embodiment of the method, the crystalline zeolite has a shape thatis orthogonal with a mid-edge length in a range of 8 μm to 20 μm. Thecrystal centered in the close-up image on the right portion of SEM (a)of FIG. 6 shows a crystalline zeolite with a mid-edge length of theoctahedron of about 14.9 μm. The crystal centered in the close-up imageon the right portion of SEM (b) of FIG. 6, where TEA (n=5), shows acrystalline zeolite with a mid-edge length of the octahedron of about11.8 μm. The crystal centered in the close-up image on the right portionof SEM (c) of FIG. 6, where TEA (n=7), shows a crystalline zeolite witha mid-edge length of the octahedron of about 9.68 μm. Note that the leftportion of micrographs SEMs (b)-(c) of FIG. 6 (TEA (n=5, 7respectively)) both show combinations of P and X crystalline zeolitesmerged (see arrows for SEMs (b)-(c) of FIG. 6). Large NaX zeolite usingTEA (n=3) do not appear to show such clustering of combined P and Xzeolites.

Calcium Ion-Exchange of NaX Zeolite

In an embodiment of the method, the step of forming the crystallinezeolite includes ion-exchanging a first crystalline zeolite with calciumions to form a second, ion-exchanged crystalline zeolite. Calcium Xzeolite (CaX) is prepared by ion-exchange with a commercial-grade NaXzeolite (not the large crystal NaX previously formed) by exchanging thesodium ions for calcium ions. The commercial-grade NaX zeolites aresmall crystallites having a mid-edge octahedral length in a range offrom about 1 to about 2 The resulting CaX zeolites are about the samesize.

About 10 g (grams) of commercial-grade NaX is constantly stirred in 200mL (milliliters) of 0.32 M (Molar) Ca(NO₃)₂ (calcium nitrate) solutionfor about 4 hours to perform the ion-exchange.

FIG. 7 is a graph showing traces of NH₃ (ammonia) temperature programmeddesorption (TPD) profiles of the ion-exchanged CaX zeolite and thecommercial-grade NaX zeolite. FIG. 7 indicates that ion-exchange of NaXzeolite with Ca²⁺ ions generate acid sites that catalyze the selectivecarbon deposition inside the zeolite micropores. These micropores aredirected towards small hydrocarbon molecules. The ion-exchanged CaXzeolite shows two desorption peaks at 473 K and 653 K, which indicatesthe presence of two types of acid sites. In contrast, thecommercial-grade NaX zeolite does not show any desorption profile, whichindicates that there are no acid sites.

The CaX zeolite also appears to have increased thermal stability. Table1 compares the thermal stability of commercial-grade NaX zeolite andcalcium-ion exchanged X zeolite.

TABLE 1 Thermal stability of NaX and ion-exchanged CaX crystal zeolites.Sample A_(z) ^([1]) T_(init) ^([2]) (°K) T_(0.5) ^([3]) (°K) NaX 1 9331043 Ca^(ex)X 0.93 983 1153 ^([1])Equivalent fraction of exchange cationin zeolite. ^([2])Temperature at which structural degradation is firstobserved from the X-ray powder pattern (K). ^([3])Temperature at whichthe structure is 50% decomposed (K).

Table 1 shows enhancement of the thermal stability of the calcium-ionexchanged X zeolite (CaX) as seen in relative increases in both T_(init)and T_(0.5). This is a benefit for performing CVD using a CaXion-exchanged zeolite over a NaX zeolite: The crystallinity of the CaXzeolite does not change even at 973 K, which is useful given that CVDtemperatures are in a range of from about 873 K to about 973 K.

Carbon Deposition within CaX Zeolite

Carbon vapor deposition (CVD) is performed using a plug-flow reactor.About one gram of zeolite (commercial-grade NaX zeolite,previously-produced ion-exchanged CaX zeolite) is placed in theplug-flow reactor. The temperature is increased within the reactor in acontrolled, gradual manner to a CVD temperature under continuous heliumflow. After stabilization at the CVD temperature for about 30 minutes,the helium gas is changed over to the organic precursor gas that is acombination of helium and organic precursor.

Three different organic precursors are used for testing three organicprecursor gases for CVD: Propylene, ethanol and acetylene. The kineticdiameters of both propylene and ethanol are 0.45 nm and acetylene is0.33 nm. For introducing propylene as the organic precursor, the organicprecursor gas has a composition of 2 vol. % propylene in a He mixture.For introducing ethanol, the organic precursor gas has a composition ofethanol-saturated helium (room temperature; 6 kPa pressure). A bubbleris used to introduce the helium through the liquid ethanol to form thesaturated gas mixture. For introducing acetylene, the organic precursorgas has a composition of 2% vol. % acetylene in a He mixture. Eachorganic precursor gas is introduced to each zeolite sample at a massflow rate of about 200 mL/minute per gram of zeolite. The organicprecursor gas is introduced and maintained at the mass flow rate for aCVD period during which the organic precursor adsorbs into andcarbonizes within the zeolite at the CVD temperature, forming acarbon-zeolite composite. After the CVD period has elapsed, theintroduced organic precursor gas is changed to the non-reactive gas(pure helium) and the plug-flow reactor is permitted to cool to roomtemperature.

Each resultant carbon-zeolite composite is treated with an aqueousstrong mineral acid mixture comprising 3.4 wt. % HCl and 3.3 wt. % HFacids. The resultant carbon-zeolite composite is exposed to the aqueousstrong mineral acid mixture twice at room temperature for a 1 hourperiod. The resultant carbon template of the zeolite—a negative replicaof the zeolite—is filtered, washed with deionized water and driedovernight at 373 K.

Effect of Organic Precursor on Forming a Carbon Template of the ZeoliteUsing the CaX Zeolite

Two different organic precursors—propylene and ethanol—are applied atdifferent CVD temperatures to form several carbon templates of thezeolite. For this experiment, the following designation code indicatesthe process used for manufacturing each carbon template of the zeolite:“zeolite template-CVD temperature/organic precursor/CVD time heattreatment”, where “zeolite template” is the ion and type of zeolitetemplate used (NaX, CaX). “CVD temperature” is in K for the four-hourperiod of organic precursor introduction. “Organic precursor” isselected from propylene (“P”), ethanol (“E”) and acetylene (“A”). “CVDtime heat treatment” indicates the length of post-CVD heat treatment at1123 K in hours. For example, “CaX-973P5” means a CaX zeolite templateat a CVD temperature of 973 K while introducing an organic precursor gascontaining propylene for a CVD period of 5 hours.

FIG. 8 is a graph showing traces of the XRD analysis for several carbontemplates of the zeolite made from a CaX zeolite. Three carbon templatesof the zeolite are formed: CaX-1073E6, CaX-973E6 and CaX-973P5. Notethat for the sake of clarity in FIG. 8 that the individual traces ofCaX-1073E6 and CaX-973E6 are offset by a fixed value of Intensity inCPS. In reality, all three traces have a similar value at 2θ=0. Thetrace for CaX-973E6 is offset by 10000 CPS; the trace for CaX-1073E6 isoffset by 15000 CPS. The XRD patterns for all three carbon templates ofthe zeolite as given in FIG. 8 show a broad peak around 2θ in a range offrom about 5° to about 6°. The broad peak in at this 2θ value indicatesthat all three carbon templates of the zeolite have a structuralmicropore arrangement that is ordered and regular. A sharp peak in2θ=5-6° range indicates that the carbon templates of the zeolite haveregularity in microform corresponding with the structural ordering of(111) plane stacking X zeolite (also known as the “FAU” zeolitestructure). This suggests that each carbon templates of the zeolitenegatively replicates the micropore structure of the sacrificialzeolite. Of the three carbon templates of the zeolites, the negativereplica formed from CaX-973P5 shows the strongest and mosthighly-resolved peak at 2θ=5-6°. The strong peak relative to the othertwo carbon templates of the zeolite indicates that the negative replicaformed by CaX-973P5 is the most accurate representation of its zeolite.

Those of ordinary skill in gas adsorption research understand andappreciate that there are several different testing procedures fordetermining the surface characteristics for carbon-zeolite composites,carbon templates of the zeolite, thermally-treated carbon templates ofthe zeolite and methane microporous carbon adsorbents. The article byWang, et al., “Experimental and Theoretical Study of Methane Adsorptionon Granular Activate Carbons”, AIChE Journal 782-788 (Vol. 58, Issue 3)(“Wang”), describes a process and an apparatus for characterizingadsorbent materials using nitrogen porosimetry at 77 K to determine thenitrogen adsorption-desorption isotherms. BET (Brunauer-Emmett-Teller)analysis provides specific surface area of the carbon templates of thezeolite as a function of the changes to relative nitrogen pressure(P/P₀) during the isothermic testing. The D-R (Dubinin-Radushkevich)equation uses the relative nitrogen pressure data for determining thevolume of each type of pore (micro- and mesopores) present on the carbontemplates of the zeolite based upon molecular stacking mechanics if thediameter of the pore is close to the working diameter of the moleculebeing adsorbed and surface adsorption within the pore if the diametersare dissimilar.

FIG. 9 is a graph showing traces of nitrogen adsorption-desorptionisotherms of the carbon templates of the zeolite. Note that for the sakeof clarity of all three traces that the isotherm trace for CaX-973E6 hasbeen off-set by an additional adsorbed amount of 200 cm³/g and theisotherm trace for CaX-1073E6 has been off-set by an additional adsorbedamount of 250 cm³/g at P/P₀=0. The carbon templates of the zeoliteformed form CaX-973P5 shows the least deviation on the return leg of theadsorption-desorption isotherm, whereas CaX-1073E6 shows the greatest.This deviation may indicate a greater amount of mesopore volume in theCaX-1073E6 carbon templates of the zeolite versus the CaX-973P5 carbontemplates of the zeolite.

FIG. 10 is a graph showing traces of the determined pore sizedistribution using the non-local density function theory (NLDFT)algorithm on the nitrogen adsorption-desorption isotherm data shown inFIG. 9. Note that for the sake of clarity in FIG. 10 that the individualtraces of CaX-1073E6 and CaX-973E6 carbon templates of the zeolite areoffset by a fixed value of dV_(p) in centimeters per gram (cm³/g). Inreality, all three traces have a similar value at W=0 nm. The trace forCaX-973E6 is offset by 0.16 cm³/g; the trace for CaX-1073E6 is offset by0.26 cm³/g. FIG. 10 shows that all three carbon templates of the zeoliteformed from CaX zeolites have dual porosity with both micropores (about1.5 to 2 nm in diameter) and mesopores (about 2 to 5 nm in diameter).Hydrogen has a kinetic diameter of 2.89 Å and methane has a kineticdiameter of 3.8 Å. The trace of carbon templates of the zeolite madefrom CaX-973P5 indicates that it has the largest total micropore volume.The trace of carbon templates of the zeolite made from CaX-1073E6indicates the largest total mesopore volume.

Table 2 provides surface area as well as micro- and mesopore volume dataon all three carbon templates of the zeolite made from CaX in additionto a carbon template of the zeolite formed from acetylene: CaX-1023A2.As shown in FIGS. 9 and 10 and given in Table 2, the four carbontemplates of the zeolite have dual porosity (both meso- andmicro-pores). The CaX zeolite only has a microporous structure;therefore, the presence of mesopores, especially at levels greater thanabout 0.40 cm³/g, indicates a less-than-desirable negative replicationof the zeolite. The presence of mesopores indicates incomplete filing ofthe zeolite micropores with the organic precursor, which leads to a morepoorly-defined replication of the pore structure.

TABLE 2 Pore structure and surface area properties of several carbontemplates of the zeolite using commercial-grade sized ion-exhanged CaXzeolite. V_(meso) V_(total) Sample S_(BET) ^([1]) (m²/g) V_(micro)^([2]) (cm³/g) (cm³/g) (cm³/g) CaX-973P5 1915 0.75 0.34 1.09 CaX-973E61596 0.58 0.48 1.06 CaX-1073E6 1826 0.65 0.66 1.31 CaX-1023A2 2567 0.950.42 1.37 ^([1])Brunauer-Emmett-Teller (BET) specific surface area.^([2])Micropore volume determined using the D-R equation.

In Table 2, the carbon template of the zeolite made from CaX-1023A2exhibits a greater surface area (2567 m²/g) than the carbon template ofthe zeolite prepared using propylene (1900 m²/g) and ethanol (average1792 m²/g). The carbon template of the zeolite made from CaX-1023A2shows the highest total pore volume (1.37 cm³/g), the highest microporevolume (0.95 cm³/g) and the highest micropore:mesopore volume ratio ofthe four samples (2.26). The carbon template of the zeolite made fromCaX-973P5 has a similar micropore:mesopore volume ratio (2.20). Inaccordance with one example, the formed methane microporous carbonadsorbent has a BET specific surface area of 2810 m²/g, and a microporevolume of 1.04 cm³/g.

Although not intending to be limited by theory, the data anddeterminations shown in FIGS. 9 and 10 as well as Table 2 suggests thatthe organic precursor—regardless of size—cannot diffuse into the zeolitemicropores greater than a certain amount the first time it isintroduced. There is a finite volume for each pore in the zeolite thatcan take a limited amount of organic precursor molecules. To maximizethe amount of carbon present in a micropore and to provide for a bettercharacterization of the surface of the zeolite (both in overall surfacearea and micropore volume), the organic precursor should have both asmall kinetic diameter to maximize the number of molecules in themicropores as well as a high ratio of carbon to other atoms (hydrogen,oxygen) such that the amount of carbon atoms in each micropore of thezeolite is maximized during CVD.

The results obtained appear to indicate that acetylene is the best ofthe three organic precursors followed closely by propylene. The carbontemplates of the zeolite synthesized using acetylene at 1023 K for 2hours shows a relatively high BET surface area (2567 m²/g) and largemicropore volume (about 1 cm³/g). Acetylene does have a smaller kineticdiameter (0.33 nm) to methane (0.38 nm). Cetylene has an optimumcarbon:hydrogen ratio (1:1) versus propylene (1:2) and ethanol (1:3 w/loxygen) and its molecular shape is linear versus having non-linear bondangles as propylene and ethanol, which makes their kinetic diametergreater.

Introducing Acetylene Organic Precursor to Large CaX Zeolites

Acetylene should be able to occupy any micropore that methane can adsorbinto; however, mesopores still formed in the carbon templates of thezeolite formed from CaX-1023A2. In addition, the use of large X zeolitetemplates may require longer diffusion times through the zeolite. Largeamounts of crystalline zeolites (>1 g), whether small or large, mayrequire techniques to maximize the opportunity of diffusion into eachzeolite with zeolite particles contacting one another and inhibitingpoints of vapor access into each structure. The use of greater CVDtemperatures (1023 K) may cause premature deposition of carbon byacetylene before full diffusion into the sacrificial zeolite. The triplebond between the two carbon atoms of acetylene already contains asignificant amount of bond energy that is fairly easy to release andpromote reaction relative to double-bonded compounds. In combinationwith a large crystal zeolite or a bed of smaller zeolites packedtogether, there may not be an adequate diffusion period at the greaterCVD temperatures to support the formation of the carbon negative replicaof the crystalline zeolite.

A method for introducing and carbonizing acetylene at a lower CVDtemperature and then thermally-treating the deposited carbon at atemperature higher than the CVD temperature but lower than a temperaturewhere graphitizing occurs (+2000° C.) increases the density of thedeposited carbon by converting loose carbon into an interconnectedcarbon matrix) within the micropores and on the surface of thesacrificial zeolite before the zeolite is removed. Reduced temperatureacetylene CVD (≤873 K) deposits the carbon within the zeolite, formingthe carbon templates of the zeolite. At a lower CVD temperature—lessthan 1000 K, and less than 900 K—the carbon deposition should occur moreuniformly than at greater CVD temperatures by preventing carbonizationbefore penetration throughout the sacrificial zeolite. Heat treating thecarbon templates of the zeolite at a greater temperature (about 1123 K)in a non-reactive gas atmosphere dehydrogenates the deposited carbonwithin the carbon-zeolite composite and increases the amount ofcarbon-carbon bonding, forming a stronger and denser composite structureof the thermally-treated carbon template of the zeolite.

For this experiment, the following designation code indicates theprocess used for manufacturing each carbon template of the zeolite andmethane microporious carbon adsorbent: “zeolite template-CVDtemperature-CVD time heat treatment”, where “zeolite template” is theion used as part of the template zeolite (Na, Ca). “CVD temperature” isin K for the four-hour period of organic precursor introduction. “CVDtime heat treatment” indicates the length of post-CVD heat treatment at1123 K in hours. If a second “H” is present, this indicates that theorganic precursor addition and post-CVD heat treatment are repeated. Ifan “L” is present before “zeolite template”, that indicates that thezeolite template is a large-crystal X zeolite synthesized with TEA (n=3)as previously described instead of using the commercial-grade sized (1-2μm) NaX or the similar sized ion-exchanged CaX zeolite. For example,“LCaX-873-4H” indicates the methane microporous carbon adsorbent is asynthesized using a large CaX zeolite with acetylene at a CVDtemperature of 873 K, a CVD period of 4 hours and is then post-CVDthermal treatment at 1123 K for four hours. “LCaX-873-4H4H” sample is asimilarly synthesized methane microporous carbon adsorbent, but theacetylene CVD application temperature and period as well as the post-CVDheat treatment are repeated a second time at similar conditions.

Table 3 shows structural properties of several carbon templates of thezeolite and methane microporous carbon adsorbents manufactured usinglarge CaX zeolites under several different CVD and post-CVD thermaltreatments. Acetylene is the organic precursor for all tests.

TABLE 3 Pore structure and surface area properties of several carbontemplates of the zeolite and methane microporous carbon adsorbentsformed using large-crystal ion-exchanged CaX (LCaX) zeolite. S_(BET)^([1]) V_(meso) V_(total) Entry Sample (m²/g) V_(micro) ^([2]) (cm³/g)(cm³/g) (cm³/g) 1 LCaX-1023-2^([3]) 2462 0.92 0.30 1.22 2LCaX-1023-2^([4]) 2156 0.83 0.43 1.26 3 LCaX-973-3^([3]) 2381 0.93 0.311.24 4 LCaX-873-4^([3]) 841 0.33 0.12 0.45 5 LCaX-873-4H^([3]) 3049 1.120.45 1.57 6 LCaX-873- 2830 1.10 0.23 1.33 4H4H^([3]) 7 LCaX-873- 28401.12 0.21 1.33 4H4H^([4]) 8 LCaX-823- 2950 1.17 0.18 1.35 9H4H^([4])^([1])Brunauer-Emmett-Teller (BET) specific surface area.^([2])Micropore volume (V_(micro)) calculated using D-R equation.^([3])1 gram zeolite used for acetylene CVD. ^([4])5 grams zeolite usedfor acetylene CVD.

Samples numbered 1, 3 and 4 in Table 3 show several interesting effectson the produced methane microporous carbon adsorbents that may have animpact upon commercial production of methane microporous carbonadsorbents using large crystal zeolites. The three aforementioned carbontemplates of the zeolite indicate that a relatively greater CVDtemperature is useful in obtaining both a greater overall BET specificsurface area and a micropore volume than lower CVD temperatures. Samplenumber 4 (LCaX-873-4) carbon templates of the zeolite has a reduced BETsurface area and microporosity compared to samples 1 and 3 even with anadditional amount of CVD period (4 hours versus 2 or 3). Although notwanting to be limited by theory, it is believed that the carbon templateof the zeolite formed using LCaX-873-4 did not sufficiently interconnectat the CVD temperature of 873 K during the 4 hour CVD period. Thisindicates that the zeolite micropores are fully filled with depositedcarbons that have some bonding but not with a significantly interlaced3-dimensional (3-D) structure. Upon removal of the sacrificial zeoliteusing the aqueous strong mineral acid mixture, the resultant carbontemplate of the zeolite structure collapsed and was otherwise unusableas a structured adsorbent.

Performing the same operation and adding a post-CVD thermal treatmentfor four hours under a helium atmosphere at 1123 K before removal of thezeolite framework (LCaX-873-4H, sample number 5) improves not only thesurface area of the methane microporous carbon adsorbent over the carbontemplate of the zeolite by a factor of 3.6 but also increases themicropore volume by a factor of 3.4 versus sample number 4. Thesefindings were unexpected and further explored as disclosed.

Table 3 shows that additional post-CVD thermal treatment of thecarbon-zeolite composite, either through a post-CVD heat treatment(sample number 5) or a secondary CVD treatment with another post-CVDheat treatment (sample numbers 6-9) when using a reduced CVD temperature(<900 K), provides a highly microporous structure in the methanemicroporous carbon adsorbent that has adequate structural integrity forremoval of the sacrificial large crystal zeolite without collapsing.

Comparing the results of methane microporous carbon adsorbent samplenumbers 6-8 with methane microporous carbon adsorbent sample number 5from Table 3, there is a reduction in the mesopore volume for samplenumbers 6-8 while comparatively maintaining the BET specific surfacearea and micropore volume. Sample numbers 6-8 have a micropore:mesoporevolume ratio in a range of from about 4.7 to about 6.5, which is animprovement over the volume ratio of about 2.5 for sample number 5.Sample number 6 (LCaX-873-4H4H) has a reduced mesopore volume (0.23cm³/g) compared to sample number 5 (LCaX-873-4H; 0.45 cm³/g) just withthe performance of a second acetylene CVD/post-CVD thermal treatmentcycle before removing the zeolite template.

Methane microporous carbon adsorbent sample number 5, LCaX-873-4H, has agreater surface area (3049 m²/g), micropore volume (1.12 cm²/g) andmicropore:mesopore volume ratio (2.49) than that of carbon template ofthe zeolite sample number 1 (LCaX-1023-2). Comparatively, this indicatesthat the reduction of the CVD temperature, lengthening the CVD periodand applying a post-CVD thermal treatment results in an improvednegative replica of the large zeolite. The methane microporous carbonadsorbent of LCaX-873-4H shows the greatest total pore volume (1.57cm³/g) of all the samples.

The result indicates that incomplete filling of zeolite micropores withthe organic precursor before the carbon is thermally deposited leads tothe formation of mesopores in the carbon templates of the zeolite. Thesequential carbon synthesis method allows a reliable means of producingand reproducing methane microporous carbon adsorbents regardless of thezeolite amount (that is, bed thickness) used. Compare sample numbers 6and 7, which use 1 gram and 5 grams of material, respectively.

Decreasing the acetylene CVD temperature to 823 K and increasing thefirst CVD period, a methane microporous carbon adsorbent with slightlyenhanced BET surface area and micropore volume is synthesized (samplenumber 8; LCaX-823-9H4H). Methane microporous carbon adsorbent samplenumbers 5-8 indicate that a CVD temperature in in a range of from about800 K to about 900 K provides an appropriate combination of bothdispersion of acetylene and carbonization not only into small amounts ofthe large CaX zeolites but also into layered beds of the sacrificialzeolites (sample numbers 7 and 8). Lengthening the CVD period within thelimited lower temperature range appears to improve the BET specificsurface and the micropore:mesopore ratio. Although not intending to belimited by theory, it is believed that the acetylene more thoroughlypenetrates into the pore structure and forming the first carbon templateof the zeolite before the first thermal treatment cycle. At a CVDtemperature less than 773 K using acetylene, the carbon template of thezeolite forms within the carbon-zeolite composite, but the processrequires a CVD period that is not practical for commercial methanemicroporous carbon adsorbent production.

FIG. 11 is a graph showing traces of nitrogen adsorption-desorptionisotherms for the LCaX zeolite, a carbon-zeolite composite,thermally-treated carbon-zeolite composites, and the resultant methanemicroporous carbon adsorbent. Isotherms for the LCaX zeolite, thecarbon-zeolite composite for Sample 4, the thermally-treatedcarbon-zeolite for Sample 5, and the methane microporous carbonadsorbent that is Sample 6 are represented. The nitrogen isotherms areperformed at different points along the carbonization and thermaltreatment process. Except for a pristine large CaX zeolite, that is,prior to carbon deposition using acetylene, the carbon template of thezeolite—both pre- and post-thermal treatment—is maintained within thezeolite; the zeolite is not removed during isothermal testing.

FIG. 11 shows pristine LCaX zeolite having the greatest adsorbed amount,but this is obvious as it has fully-open zeolite pores. Sample number 4(LCaX-873-4) carbon-zeolite composite shows that after a CVD period of 4hours at a CVD temperature of 873 K using acetylene, the adsorptioncapacity of the carbon-zeolite composite is reduced by about 80%(measuring at P/P₀=1). The adsorption amount drops because the pores ofthe zeolite are clogged with deposition carbon in the carbon-zeolitecomposite. Sample number 5 (LCaX-873-4H) is a thermally-treatedcarbon-zeolite composite that is thermal treated at 1123 K for fourhours under a helium atmosphere. The process appears to have regeneratedabout 25% of the LCaX zeolite micropore volume. The thermal treatmentalso appears to have increased the density of the carbon template of thezeolite within the thermally-treated carbon-zeolite composite. Thethermally-treated carbon template of the zeolite forms a moreinterlinked carbon-carbon structure, which causes the carbon networkwithin the zeolite to shrink. This permits more nitrogen to penetrateinto the thermally-treated carbon-zeolite template (LCaX-873-4H).

Because many of the zeolite micropores are regenerated after thepost-CVD thermal treatment (the deposited carbon dehydrogenates and thenetwork of interlaced carbons physically shrinks as carbon-carbonbonding becomes more prevalent), a second acetylene CVD/post-CVD thermaltreatment cycle penetrates the carbon-zeolite composite and fills thenewly exposed and remaining micropores. After performing the secondacetylene CVD/post-CVD thermal treatment cycle, the micropores of thecarbon-zeolite composite sample number 6 (LCaX-873-4H4H) are almostfilled with the thermally-treated carbon template of the zeolite. Usingthe data previously presented in Table 3, the micropore:mesopore volumeratio is greater than 4 for LCaX-873-4H4H.

FIGS. 12-14 show analysis of two types of the methane microporous carbonadsorbents and two carbon templates of the zeolite using LCaX as thezeolite template and acetylene as the organic precursor. Each is madeusing 5 grams of the LCaX zeolite. FIG. 12 is a graph showing traces ofnitrogen adsorption-desorption isotherms for carbon templates of thezeolite made from LCaX-1023-2 and LCaX-873-4 and two methane microporouscarbon adsorbents made from LCaX-873-4H and LCaX-873-4H4H. FIG. 13 is agraph showing traces of the determined pore size distribution using thenon-local density function theory (NLDFT) algorithm on the nitrogenadsorption-desorption isotherm data shown in FIG. 12. Note that for thesake of clarity in FIG. 13 that the individual traces of carbon templateof the zeolite using LCaX-873-4, and the methane microporous carbonadsorbents LCaX-873-4H and LCaX-873-4H4H, are offset by a fixed value ofdV_(p) in cm³/g. In reality, all four traces have a similar value at W=0nm. The trace for LCaX-873-4 is offset by 0.30 cm³/g; the trace forLCaX-873-4H is offset by 0.43 cm³/g; the trace for LCaX-873-4H4H isoffset by 0.85 cm³/g. FIG. 14 is a graph showing traces of the XRDanalysis for carbon templates of the zeolite made from LCaX-1023-2 andLCaX-873-4 and methane microporous carbon adsorbents made fromLCaX-873-4H and LCaX-873-4H4H. Note that for the sake of clarity in FIG.14 that the individual traces of LCaX-873-4, LCaX-873-4H andLCaX-873-4H4H offset by a fixed value of Intensity in CPS. In reality,all three traces have a similar value at 2θ=0. The trace for LCaX-873-4is offset by 30000 CPS; the trace for LCaX-873-4H is offset by 50000CPS; the trace for LCaX-873-4H4H is offset by 72000 CPS.

The most precise negative replica of the LCaX zeolite structure(LCaX-873-4H4H) appears to show a classic Type I isotherm and nearingsaturation at a reduced nitrogen partial pressure (P/P₀>0.1) in FIG. 12.The LCaX zeolite with no deposited carbon shows a similar Type Iisotherm curve in FIG. 11. The methane microporous carbon adsorbentformed by LCaX-873-4H, which is synthesized in a single cycle ofacetylene CVD/post-CVD thermal treatment, shows a comparatively greatertotal pore volume than the double-cycled LCaX-873-4H4H methanemicroporous carbon adsorbent in FIG. 12. FIG. 12 shows that a largeamount of adsorption for the LCaX-873-4H methane microporous carbonadsorbent occurs in a partial pressure range of P/P₀>0.1. FIG. 13confirms that the increased adsorption amount by LCaX-873-4H methanemicroporous carbon adsorbent is due to the presence of additional porevolume in the mesopore range (the rounded hump that spreads along thetrack at values >2 nm, which indicates pore sizes outside of themicropore range)

The methane microporous carbon adsorbents (LCaX-873-4H andLCaX-873-4H4H) show in FIG. 13 a large spike (narrow, intense) in poresize distribution in the micropore regime (W<2 nm). FIG. 14 shows thatthe carbon templates of the zeolite and methane microporous carbonadsorbents that performed well either have a great CVD temperature andno post-CVD treatment (LCaX-1023-2) or have at least one post-CVDthermal treatment cycle (LCaX-873-4H, LCaX-873-4H4H) show a response inintensity at about 20=6.3° in the XRD trace. The methane microporouscarbon adsorbent from LCaX-873-4H4H shows a very sharp peak at thisvalue. This indicates that the adsorbent has an ordered microporousstructure very similar to the template zeolite (see FIG. 5 for the largeNaX zeolite having a TEA (n=3); FIG. 8 for a similar bump for thecommercial-grade sized ion-exchanged CaX replicas). The presence of thesharp XRD intensity peak at 20=6.3° is useful for indicating theprecision of the negative replication of the sacrificial zeolitestructure (that is, efficiency of carbon deposition and thermaltreatment).

Forming Methane Microporous Carbon Adsorbents from Commercial BEA andCommercial-Grade Sized CaX Zeolites

A commercial BEA zeolite is obtained from Zeolyst Int'l (Conshohocken,Pa.) having a Si:Al molar ratio of about 19. The BEA zeolite is around-shaped particle with a size distribution in a range of from about500 nm to about 1 μm. A CaX zeolite (commercial-grade sized (1-2 μm)Ca⁺² ion-exchanged NaX zeolite) is also used and is manufactured aspreviously described. Each zeolite goes through a similar sequentialcarbon synthesis method: a first CVD process using acetylene at a CVDtemperature of 823 K for a first CVD period of 9 hours, a first post-CVDthermal treatment in a helium atmosphere at 1123 K for four hours, asecond CVD with acetylene at a CVD temperature of 823 K for a second CVDperiod of 4 hours, a second post-CVD thermal treatment in a heliumatmosphere at 1123 K for four hours. The sacrificial zeolite frameworksare etched away in several aqueous strong mineral acid mixture washes.The resultant methane microporous carbon adsorbents are recovered.Testing on the methane microporous carbon adsorbents are presented inTable 4.

TABLE 4 Pore structure and surface area properties of the methanemicroporous carbon adsorbents formed from BEA and commercial-grade sizedCaX zeolites. V_(meso) V_(total) Sample S_(BET) ^([1]) (m²/g) V_(micro)^([2]) (cm³/g) (cm³/ g) (cm³/g) CaX-823-9H4H 2933 1.18 0.28 1.46BEA-823-9H4H 2940 1.19 0.31 1.50 ^([1])Brunauer-Emmett-Teller (BET)specific surface area. ^([2])Micropore volume (V_(micro)) calculatedusing D-R equation.

FIG. 15 shows SEMs of (a) BEA crystalline zeolite and (b) methanemicroporous carbon adsorbents made using the BEA zeolite. (a) of FIG. 15is a SEM of the commercially-obtained BEA zeolite. (b) of FIG. 15 is aSEM of the methane microporous carbon adsorbents made using thecommercially-obtained BEA zeolite of SEM (a).

Both methane microporous carbon adsorbents given Table 4 show high BETspecific surface area (about 3000 m²/g) as well as micropore volume(about 1.2 cm³/g). The methane microporous carbon adsorbent formed fromCaX has a micropore:mesopore volume ratio of 4.21. The methanemicroporous carbon adsorbent formed from BEA has a micropore:mesoporevolume ratio of 3.84.

FIGS. 16 and 17 show analysis of both types of methane microporouscarbon adsorbent given in Table 4. FIG. 16 is a graph showing traces ofnitrogen adsorption-desorption isotherms for the methane microporouscarbon adsorbent made from CaX and BEA zeolites. Both materials showType I N₂ adsorption-desorption isotherms. FIG. 17 is a graph showingtraces of the XRD analysis for the methane microporous carbon adsorbentmade from the CaX and BEA zeolites. Note that for the sake of clarity inFIG. 17 that the individual trace of CaX-823-9H4H is offset by a fixedvalue of Intensity in CPS. In reality, the traces have a similar valueat 2θ=0. The trace for CaX-823-9H4H is offset by 40000 CPS. FIG. 17shows very sharp XRD peak in the low angle regime (2θ<10°). Both FIGS.17 and 18 as well as the ratio of micropore:mesopore volume eachindicate that the methane microporous carbon adsorbents are negativereplicas that closely resemble each of their sacrificial zeolite inordered micropore structure.

FIG. 18 is a graph showing traces of the determined pore sizedistribution using the NLDFT algorithm for four methane microporouscarbon adsorbents formed using two acetylene CVD/post-CVD thermaltreatment cycles. Note that for the sake of clarity in FIG. 18 that theindividual traces for CaX-823-9H4H, CaX-873-4H4H and LCaX-823-9H4H areoffset by a fixed value of dV_(p) in cm³/g. In reality, all four traceshave a similar value at W=0 nm. The trace for CaX-823-9H4H is offset by0.22 cm³/g; the trace for CaX-873-4H4H is offset by 0.50 cm³/g; thetrace for LCaX-823-9H4H is offset by 0.90 cm³/g. FIG. 18 shows fourdifferent negative replicas using three different sacrificial zeolites,two different CVD temperatures, and two different first CVD periods.FIG. 18 demonstrates is that all four methods—even with different smallpore zeolites, CVD temperature and CVD period—show a very narrow poresize distribution in the micropore range. FIG. 18 also shows that themesopore pore range for these methane microporous carbon adsorbent isrelatively insignificant.

The results of this experiment indicate that other crystalline zeolitestructures may be used as sacrificial templates for forming the methanemicroporous carbon adsorbent. The experiments have shown that NaX, CaXand BEA zeolites are useful in forming the methane microporous carbonadsorbent. FAU, which include the commercial-grade sized NaX, the largeNaX (LNaX), the large and commercial-grade sized ion-exchanged NaX (CaXand LCaX), and NaY; EMT, which is similar to FAU; and BEA zeolitestructures are all 12-membered ring structures and have 3-dimensionalpore connectivity, which are suitable to act as the framework forforming the 3-dimensional negative replica. In an embodiment of themethod, the crystalline zeolite is selected from the group consisting ofFAU, EMT and BEA zeolite structures. In an embodiment of the methanemicroporous carbon adsorbent, the shape is in the form of the negativereplica of a crystalline zeolite that is selected from the groupconsisting of FAU, EMT, BEA zeolite structures, and combinations of thezeolite structures.

Comparative Methane Adsorption for Several Carbon Adsorbents

Wang provides a description of the testing procedures and the apparatusfor determining the gravimetric basis for adsorption isotherms. FIG. 19is a graph showing several traces of the methane adsorption isotherms ona gravimetric basis for a carbon template of the zeolite and severalmethane microporous carbon adsorbents at 298 K. Each carbon template ofthe zeolite and the methane microporous carbon adsorbents aremanufactured using the code provided in FIG. 19 described supra. Theevaluation pressure range is from about 0 to about 40 bar. Theevaluation temperature is maintained at 298 K. The value provided at theend of each isotherm trace is the “CH₄ stored” value. As shown in FIG.19, the methane microporous carbon adsorbent from CaX-823-9H4H, whichdoes not use large crystal zeolite, has the greatest methane storedvalue on a weight basis. The greater CVD temperature with nopost-thermal treatment carbon template of the zeolite—CaX-1023-2—has arelatively reduced methane stored value at 40 bar pressure compared tothe methane microporous carbon adsorbents.

Table 5 shows the storage properties of the carbon template of thezeolite, the five methane microporous carbon adsorbents, and two knowncommercial activated carbon adsorbents. “Maxsorb® 3000” (Kansai Coke andChemicals Co., Ltd; Japan) is a carbon material (about 3000 m²/g) thatis activated by exposure to a solution of potassium hydroxide (KOH).“SRD-08016” is an activated powdered carbon material supplied fromChemviron Carbon (Feluy, Belgium).

TABLE 5 Pore structure and surface area properties as well as determinedmethane adsorption properties of several commercial activated carbonmaterials, a carbon template of a zeolite, and several methanemicroporous carbon adsorbents. CH₄ CH₄ CH₄ CH₄ CH₄ S_(BET) V_(micro)stored retained 1 deliv. deliv. deliv. Sample (m²/g) (cm³/g) (mg/g) bar(wt. %) (mg/g) (v/v)^([1]) (v/v)^([2]) Maxsorb ® 3000 3180 1.31 180 10162 102 89 SRD-08016 1840 0.74 124 16 104 86 76 CaX-1023-2 2567 0.95 16112 142 103 71 CaX-823-9H4H 2933 1.15 192 11 171 104 73 CaX-873-4H4H 26311.06 172 11 152 109 67 LCaX-823-9H4H 2950 1.17 180 11 160 122 103LCaX-873-4H 3049 1.12 174 12 153 105 73 LCaX-873-4H4H 2840 1.10 184 11164 123 103 ^([1])Delivered CH₄ amount calculated based on packingdensity. ^([2])Delivered CH₄ amount calculated based on tap density.

FIG. 20 shows SEMs (a)-(b) of methane microporous carbon adsorbents madeusing calcium-ion substituted X zeolites. (a) of FIG. 20 is a SEM ofmethane microporous carbon adsorbents made using a NaX calcium-ionsubstituted zeolite (CaX). (b) of FIG. 20 is a SEM of methanemicroporous carbon adsorbents made using an LNaX calcium-ion substitutedzeolite (LCaX). In an embodiment of the adsorbent, the shape isorthogonal with a mid-edge length in a range of 8 μm to 20 μm. As bothSEMs (a)-(b) of FIG. 20 show, the ion-exchange of Ca²⁺ in the NaX andLNaX zeolites did not affect the octahedral particle morphology of theresultant methane microporous carbon adsorbents formed from CaX andLCaX. Instead, it appears that on the “macro” level to be a negativereplica of the sacrificial crystalline zeolite.

Table 5 shows that Maxsorb®, the carbon template of the zeolite, and thefive methane microporous carbon adsorbents retain an amount of methanein a range of from about 10 wt. % to about 12 wt. % residual amount ofCH₄ at 1 bar. The “delivered CH₄” amount represents the amount ofmethane that is adsorbed and released between cycles of 1 bar and 40bar, and is determined by subtracting the adsorption amount detected at1 bar from the adsorption amount detected at 40 bar.

Both packing and tap densities are used for calculating the volumetricCH₄ adsorption amounts of the methane microporous carbon adsorbents. Thefive methane microporous carbon adsorbents and the carbon template of azeolite given in Table 5 show similar methane adsorption amounts on agravimetric basis (FIG. 19), but the determined volumetric values aredifferent due to deviations in packing and tap densities. Deviationsappear more significant for tap density. Although not intending to belimited by theory, the methane microporous carbon adsorbents formingfrom the large sacrificial zeolite particles (LCaX series) show greateroverall methane adsorption volume capacity than the materials createdfrom the smaller sacrificial zeolite particles (CaX series). On a gasvolume basis, therefore, the LCaX formed methane microporous carbonadsorbents that provide greater methane adsorption even at a reducedvolume density than the CaX formed methane microporous carbon adsorbentsand the carbon template of the zeolite.

Methane microporous carbon adsorbents manufactured using LCaX-823-9H4Hand LCaX-873-4H4H according to Table 5 have a greater methane adsorptionvolume capacity in a range of from about 10 vol. % to about a 20 vol. %on either a packing density or a tap density basis compared to Maxsorb®3000.

Forming Negative Carbon Replicas of Commercial NaX Zeolite

Table 1 shows that a commercial-grade NaX zeolite has a lower thermalstability than an ion-exchanged CaX zeolite (T_(init)), and thediscussion regarding Table 1 indicates that NaX zeolites may beunsuitable for forming methane microporous carbon adsorbents. Using asequential carbon synthesis method having a CVD temperature that is lessthan 900 K, however, provides an opportunity to reexamine thisassumption. Table 6 shows two methane microporous carbon adsorbents: Onemade with CaX zeolite and one made with NaX zeolite.

TABLE 6 Pore structure and surface area properties of two methanemicroporous carbon adsorbents formed from commercial-grade sized NaX andCaX zeolites. S_(BET) ^([1]) V_(micro) ^([2]) V_(meso) V_(total) Sample(m²/g (cm³/g) (cm³/g) (cm³/g) CaX-823-9H4H 2933 1.18 0.28 1.46NaX-823-4H2H 2974 1.18 0.23 1.41 ^([1])Brunauer-Emmett-Teller (BET)specific surface area. ^([2])Micropore volume (V_(micro)) calculatedusing D-R equation.

The post-CVD thermal treatment is performed twice for four hours at 1123K on the NaX zeolite. Although not wanting to be limited by theory, itis believed that the deposited carbon structure within the NaX zeoliteafter CVD has sufficient strength to support the carbon-NaX zeolitecomposite even during the thermal treatment post-CVD process such thatthe NaX zeolite framework remains intact and does not degrade. Theconnected thermally-treated carbons in and between the micropores of theNaX zeolite internally stabilizes the zeolite structure while the carbonbecomes denser during the post-CVD treatment process. Table 6 shows thatthe methane microporous carbon adsorbent from the NaX zeolite is veryclose to the 3000 m²/g BET specific surface area value that one ofordinary skill in the art may describes as a “super adsorbent” (≥3000m²/g).

Based upon the data presented in Tables 1-6 and FIGS. 2-22, in anembodiment of the methane microporous carbon adsorbent the BET specificsurface area is in a range of from about 2500 m²/g to about 3100 m²/g.In an embodiment of the adsorbent, the micropore volume is in a range offrom 0.95 cm³/g to 1.19 cm³/g as determined by the Dubinin-Radushkevichequation. In an embodiment of the adsorbent, the micropore to mesoporevolume ratio is in a range of from 4 to 6. In an embodiment of theadsorbent, the stored methane value is in a range of from 172 mg/g to192 mg/g. In an embodiment of the adsorbent, the methane delivered valueis a range of from 152 mg/g to 171 mg/g in a pressure range from 1 barto 40 bar.

FIGS. 21 and 22 show analysis of both types of methane microporouscarbon adsorbents given in Table 6. FIG. 21 is a graph showing traces ofnitrogen adsorption-desorption isotherms for the methane microporouscarbon adsorbents formed from CaX and NaX zeolites. Both methanemicroporous carbon adsorbents show Type I N₂ adsorption-desorptionisotherms. FIG. 22 is a graph showing traces of the determined pore sizedistribution using the non-local density function theory (NLDFT)algorithm on the nitrogen adsorption-desorption isotherm data shown inFIG. 21. Note that for the sake of clarity in FIG. 22 that theindividual trace of NaX-823-4H2H is offset by a fixed value of dV_(p) incm³/g. In reality, the traces have the same value at W=0. FIG. The tracefor NaX-823-4H2H is offset by 0.25 cm³/g. In reality, all traces havethe same value at 2θ=0. FIG. 22 shows the methane microporous carbonadsorbent made from the NaX zeolite having a strong spike in the rangeof from about 1 nm to about 2 nm pore width. FIG. The micropore:mesoporevolume ratio for the methane microporous carbon adsorbents using the NaXzeolite is about 5.13, which is within the range of methane microporouscarbon adsorbents made from CaX and LCaX given in Table 3 and discussedsupra.

FIG. 23 shows SEMs (a)-(b) of methane microporous carbon adsorbents madeusing sodium X zeolites. (a) of FIG. 23 is a SEM of methane microporouscarbon adsorbents made using a commercial-grade sized NaX zeolite. (b)of FIG. 23 is a SEM of methane microporous carbon adsorbents made usinga LNaX zeolite. The size difference of the zeolite material does notaffect the octahedral particle morphology. In comparing (a)-(c) of FIG.6 with (a)-(b) of FIG. 23, one can observe that the change in size doesnot affect the octahedral shape or the ability to adsorb the organicprecursor for forming the methane microporous carbon adsorbents.

Scaled Synthesis of Methane Microporous Carbon Adsorbents UsingCommercial-Grade Size NaX Zeolite

The rotary tubular furnace shown in FIG. 2 is used to perform scaledsynthesis of methane microporous carbon adsorbents for amounts greaterthan 1-5 grams from commercial-grade NaX zeolites. The size of thecommercial-grade NaX zeolites is such that the mid-edge length is about2 μm. About 50 grams of the commercial-grade sized NaX zeolite and about50 grams of cleansed sea sand (washed in deionized water; particle sizesof about 15 to about 20 mesh) are introduced into the cylindricalstainless-steel container located in the center of the tubular furnace.The sea sand is used to help solids mixing and to keep the NaX zeolitesfrom sticking together during carbon vapor deposition and thermaltreatment. The cylindrical container is purged with argon (anon-reactive gas), the cylindrical container rotated and the temperaturewithin the cylindrical container ramped up to 823 K. As the containerrotates, the NaX zeolite and the sea sand particles not only collidewith one another but also the internal baffles of the cylindricalcontainer, causing the zeolite crystals to be dropped through theatmosphere contained within the cylindrical container every few seconds.Upon reaching the first CVD temperature of 823 K, an organic precursorgas that is a mixture of 4 vol. % acetylene in argon (Ar) is introducedinto the cylindrical stainless-steel container at a flow rate of about1000 mL/minute and maintained for a first CVD period of about 7 hours.After the first CVD period elapses, the introduced gas is switched fromthe acetylene/Ar gas mixture to pure Ar, which is introduced at a rateof 500 mL/minute. The temperature is ramped up to the post-CVD thermaltreatment temperature of 1123 K and maintained at the thermal treatmentconditions for about three hours. After three hours of post-CVD thermaltreatment, the cylindrical container is permitted to cool partially downunder Ar flow until reaching 823 K. Upon reaching the second CVDtemperature of 823 K, the organic gas mixture of 4 vol. % acetylene inAr is reintroduced into the cylindrical stainless-steel container at aflow rate of about 1000 mL/minute and maintained at that rate for asecond CVD period of about 4 hours. After the second CVD period elapses,the introduced gas is switched from the organic precursor gas ofacetylene/Ar mixture to pure Ar, which is introduced at a rate of 500mL/minute. The temperature is ramped up to the second treatmenttemperature of 1123 K for the second post-CVD thermal treatment andmaintained at the treatment temperature for three hours. After threehours of the second post-CVD thermal treatment, the cylindricalcontainer is permitted to cool down under Ar flow until reaching roomtemperature. Upon reaching room temperature, the rotating cylinder ishalted and the thermally-treated carbon-zeolite composite is collected.The thermally-treated carbon-zeolite composite is separated from the seasand using a sieve, and the NaX zeolite is removed from thethermally-treated carbon-zeolite composite using the aqueous strongmineral acid mixture containing HCl/HF previously. The recovered methanemicroporous carbon adsorbents are tested for surface and pore propertiesas well as for comparative isothermal information.

TABLE 7 Pore structure and surface area properties of two methanemicroporous carbon adsorbents formed from commercial-grade sized NaX (1gram) and scaled synthesis from commercial-grade sized NaX (50 grams).V_(micro) ^([2]) V_(meso) V_(total) Sample S_(BET) ^([1]) (m²/g) (cm³/g)(cm³/g) (cm³/g) NaX-823-4H2H 2980 1.18 0.23 1.41 NaX-large scale 28101.04 0.39 1.43 synthesis Brunauer-Emmett-Teller (BET) specific surfacearea. ^([2])Micropore volume (V_(micro)) calculated using D-R equation.

FIG. 24 is a graph showing traces of nitrogen adsorption-desorptionisotherms for the NaX and the mass produced NaX methane microporouscarbon adsorbents. As shown in Table 7, the methane microporous carbonadsorbent synthesized in a scaled synthesis process (50 grams of NaXzeolite) shows only a slight reduction in surface area and microporevolume versus the methane microporous carbon adsorbent synthesized using1 g NaX zeolite in the plug-flow reactor (NaX-823-4H2H).

What is claimed is:
 1. A sequential carbon synthesis method for forminga methane microporous carbon adsorbent, the method comprising:introducing an organic precursor gas comprising an organic precursor fora chemical vapor deposition (CVD) period to a crystalline zeolite thatis maintained at a CVD temperature such that a carbon-zeolite compositeforms, wherein the introduced organic precursor adsorbs via CVD into thecrystalline zeolite, the organic precursor converts into carbon withinthe crystalline zeolite forming a carbon template of a zeolite;introducing a non-reactive gas for a thermal treatment period to thecarbon-zeolite composite maintained at a thermal treatment temperaturesuch that the thermally-treated carbon-zeolite composite forms, whereinthe carbon template of the zeolite within the crystalline zeoliteconverts into a thermally-treated carbon template of the zeolite; andintroducing an aqueous strong mineral acid mixture to thethermally-treated carbon-zeolite composite such that crystalline zeolitedissolves and the methane microporous carbon adsorbent forms, whereinthe methane microporous carbon adsorbent is a negative replica of thecrystalline zeolite, has a BET specific surface area, a microporevolume, a micropore to mesopore volume ratio, a stored methane value anda methane delivered value; and forming the crystalline zeolite, whereinforming the crystalline zeolite comprises ion-exchanging a firstcrystalline zeolite with calcium ions to form a second crystallinezeolite, wherein the crystalline zeolite comprises tri-ethanolamine(TEA), and wherein the shape of the crystalline zeolite has anoctahedral geometry such that an edge length of the octahedron is in arange of 8 μm to 20 μm.
 2. The method of claim 1, wherein the organicprecursor is selected from the group consisting of acetylene, ethylene,propylene, ethanol and combinations thereof.
 3. The method of claim 1,wherein the organic precursor gas further comprises the non-reactivegas.
 4. The method of claim 1, wherein the CVD period is in a range offrom 2 hours to 9 hours.
 5. The method of claim 1, wherein the CVDtemperature is in a range of from 800 K to 900 K.
 6. The method of claim1, wherein the non-reactive gas is selected from the group consisting ofhelium, argon and combinations thereof.
 7. The method of claim 1,wherein the thermal treatment period is in a range of from 2 hours to 4hours.
 8. The method of claim 1, wherein the thermal treatmenttemperature is in a range of from 1100 K to 1200 K.
 9. The method ofclaim 1, wherein the strong mineral acid is selected from the groupconsisting of hydrochloric acid (HCl), hydrofluoric acid (HF) andcombinations thereof.
 10. The method of claim 1, wherein the crystallinezeolite is selected from the group consisting of FAU, EMT, BEA zeolitestructures, and combinations of the zeolite structures thereof.
 11. Themethod of claim 1, further comprising: forming the crystalline zeolite.12. The method of claim 1, wherein the sequential carbon synthesis ofthe crystalline zeolite to form the methane microporous carbon adsorbentis carried out using a large-scale, continuous batch process using arotary tubular furnace.
 13. The method of claim 12, wherein 50 grams ofa commercial grade crystalline zeolite is used to form the methanemicroporous carbon adsorbent.
 14. The method of claim 1, furthercomprising: introducing the organic precursor gas comprising the organicprecursor for a second CVD period to the thermally-treatedcarbon-zeolite composite that is maintained at a second CVD temperaturesuch that a second carbon-zeolite composite forms, wherein the organicprecursor adsorbs via CVD into the thermally-treated carbon-zeolitecomposite, the organic precursor converts into carbon within thethermally-treated carbon-zeolite composite and both the carbon and thethermally-treated carbon-zeolite composite form a second carbon templateof a zeolite; and introducing the non-reactive gas for a second thermaltreatment period to the second carbon-zeolite composite maintained at asecond thermal treatment temperature such that a secondthermally-treated carbon-zeolite composite forms, wherein the secondcarbon template of the zeolite within the second carbon-zeolitecomposite converts into a second thermally-treated carbon-zeolitecomposite; wherein both the introducing the organic precursor gas andthe introducing the non-reactive gas occur before the introduction ofthe aqueous strong mineral acid mixture, and wherein the aqueous strongmineral acid mixture is introduced to the second thermally-treatedcarbon-zeolite composite instead of the thermally-treated carbon-zeolitecomposite.
 15. The method of claim 14, wherein the CVD temperature andthe second CVD temperature are the same.
 16. The method of claim 14,wherein the CVD period and the second CVD period are the same.
 17. Asequential carbon synthesis method for forming a methane microporouscarbon adsorbent, the method comprising: introducing an organicprecursor gas comprising an organic precursor for a chemical vapordeposition (CVD) period to a crystalline zeolite that is maintained at aCVD temperature such that a carbon-zeolite composite forms, wherein theintroduced organic precursor adsorbs via CVD into the crystallinezeolite, the organic precursor converts into carbon within thecrystalline zeolite forming a carbon template of a zeolite; introducinga non-reactive gas for a thermal treatment period to the carbon-zeolitecomposite maintained at a thermal treatment temperature such that thethermally-treated carbon-zeolite composite forms, wherein the carbontemplate of the zeolite within the crystalline zeolite converts into athermally-treated carbon template of the zeolite; introducing an aqueousstrong mineral acid mixture to the thermally-treated carbon-zeolitecomposite such that crystalline zeolite dissolves and the methanemicroporous carbon adsorbent forms, wherein the methane microporouscarbon adsorbent is a negative replica of the crystalline zeolite, has aBET specific surface area, a micropore volume, a micropore to mesoporevolume ratio, a stored methane value and a methane delivered value; andforming the crystalline zeolite, wherein forming the crystalline zeolitecomprises ion-exchanging a first crystalline zeolite with calcium ionsto form a second crystalline zeolite, wherein the formed methanemicroporous carbon adsorbent has a BET specific surface area of1596-3049 m²/g, a micropore volume of 0.58-1.12 cm³/g, a micropore tomesopore volume ratio of 1.21-3.07, a stored methane value of 142-153mg/g, and a methane delivered value of 103-105 mg/g.
 18. The method ofclaim 17, wherein the formed methane microporous carbon adsorbent has aBET specific surface area of 2810 m²/g, and a micropore volume of 1.04cm³/g.